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The remnants of "Bill" are showing up very well on the surface analysis this morning:
Note, due to the large number of maps to look at, I am using imbedded links for this post, rather than inline images. So be sure to follow the links to see the various maps being discussed.
12pm: An animation of the radar for the afternoon and evening: HERE
9pm: Still lots of warnings out for areas to the east of the DC area and areas to the north, up in south-central PA. Reports of 1 inch hail in Fredericksburg VA, 1 inch hail in Burtonsville MD, 3/4 inch hail in Fairland MD, 3/4 inch hail in Westover VA (near Arlington), 1 inch hail in Berryville WV, and 3/4 inch hail in Fort Ashby WV. In addition, there were numerous reports from the south central region of PA of hail from an inch to 1.5 inches. There was also a report of a funnel cloud near Altoona at around 7:15pm (not far from the mesolow).
The 8pm surface analysis continues to show a mesolow just to our north. The 8pm pressure change analysis shows a fairly distinct pressure fall/rise couplet associated with the low. Looking at the 8pm surface streamline analysis, the front is pretty close to the model forecast position for this time, although there is not quite as distinct a wind shift line as the model was implying.
My nowcasting of this event depended on understanding the approaching jet streak and its likely effect when it reached our area. It got here fairly late in the day, but there was still sufficient instability to get some updrafts going.
Here's some lightning photos I took around 8pm. Not bad for hand-held LOL:
8pm: An impressive thunderstorm with lots of lightning and some small hail hit at my location (Falls Church) around 7:45pm and lasted about 20 minutes.
7pm: The 7pm surface analysis shows a mesolow over the WV panhandle. Many storms have formed in that area in the last couple of hours, accompanied by a variety of warnings. There's also a few storms approaching the immediate DC area but no severe reports so far from those. (Here's the 7pm visible satellite and radar)
4pm: SPC issued a tornado watch for our area about an hour ago. The 4pm visible satellite and radar shows the cirrus (possible jet streak) moving north rapidly into south central VA. Showers are starting to pop up around our area and also to the west over the ridge lines. IMO the area of greatest probabability for severe in the next hour or two is southwest and south central VA. For our area, the timing suggests maybe 7 or 8 pm for the greatest chance of severe.
2pm: The current visible satellite and radar shows things are getting just a bit bubbly west of the blue ridge. There is also a cirrus shield moving in from the south, seen over the Carolina's. I'd suspect these cirrus clouds are a manifestation of the 300mb jet streak nosing in from the southwest, and the storms forming over the Carolina mountains would be in the LFQ of that jet streak.
There should be enough surface heating to see some convection start up soon, and the ridge lines will likely be the initial focus. If the LFQ of the 300mb jet streak interacts favorably with the front later today, things could get very interesting indeed.
1pm: SPC has expanded the Enhanced region a bit with their midday update. Temperature and dewpoint continue to rise at Dulles with a noon temperature of 70 and dewpoint of 61. A look at the 12Z GFS run shows our area under broad cyclonic flow at 500mb with a closed upper low near the Great Lakes. No strong upper short waves are to be noted, but there are some weaker vorticity maxes rotating around the low. Some interesting things I noticed about the 12Z GFS:
-- Although there is no well defined short wave at 500mb, the models
are forecasting fairly strong
for later today.
-- The forecast of surface streamlines for this evening shows a well defined wind shift line moving through our region.
-- The model forecast of surface-based cape for this afternoone is reasonably high.
-- A strong 300mb jet is forecast to nose into our area later today, as shown by the model forecast of 300mb wind speed.
It appears that the strengthening frontal boundary will be the focus
for thunderstorm development later today, enhanced by:
1) increasing instability due to surface heating and southerly moist flow
2) 500mb height falls
3) 300mb jet streak dynamics
11am: Earlier this morning SPC upgraded much of our area to an "Enhanced" risk of severe, which is their new risk category between "slight" and "moderate". The 11am visible satellite shows lots of sun over our area. The map of 11am SLP change indicates there is a front just west of the mountains. The map of 11am surface streamlines also shows a frontal boundary in that area, but the lack of any distinct wind shift line indicates the front is diffuse at this time. The models are indicating this front will strengthen today as it moves towards us. The morning sounding from Dulles, as plotted by SPC shows plenty of moisture below 700mb. Although not particularly unstable at 12Z, the increase currently taking place in surface temperature and dewpoint will result in a more unstable sounding. New model data, and an SPC update, are expected around 12:30pm local time.
9am: The 8am surface chart shows lower pressure far to the northwest, with a pressure gradient over our area that will encourage low level southerly wind flow. The areas near DC to the south and east of the I-95 corridor are very rich with moisture -- dewpoints in the mid 60s. To the north and west, the air is somewhat cooler and drier. It appears the cold air damming from yesterday has not entirely scoured out. But satellite shows some breaks in the clouds, so the sun should do its thing as the morning progresses. The amount of sun this morning will be very important for determining how much severe we get later.
8am: SPC has our area outlined for a slight risk of severe today. The warm-frontal rains moved through last night, and you can watch an animation of that HERE. I'll be making comments through the day, so check back.
The NWS office in Chicago has finalized their report on the April 9 tornado outbreak in north central Illinois, including the EF4 Rochelle tornado. Lots of good info if you follow the link HERE
A snowy thanksgiving in the mountains....
Snow is winding down. 18 inches now, so the snow is settling some. Time to dig out.
19 inches at Noon.
Still S+, 1/8 mile visibility, 8 inches on the grass.
Getting light here. 5 inches at 7am. Photo below taken at first light. Some of the large aggregate flakes are over an inch in diameter. 7am ob at Martinsburg still rain.
I haven't had anything to add to the various NWS discussions for the winter storm that is now underway. How the storm evolves, and who gets how much snow, is now a nowcasting problem.
I am west of Martinsburg in the eastern panhandle of WV, where at 630am we are getting S+ with visibiity about 1/8 mile. The snow is falling as large aggregate flakes and very fluffy. I would estimate 2 or 3 inches accumulation already.
The radar shows my location under a heavier band of precip on the western edge of the precip field. Martinsburg is reporting rain at 6am, so the snow in this system is currently confined to the northwestern edge of the precip shield, and also at somewhat higher elevations (I am at about 1200 ft). I anticipate the 7am ob at Martinsburg to be snow.
The cold front has gone through and shifted our winds and cleared our skies, but we haven't seen much of a change of air mass as yet. The dense cold air that dropped down from the high plains has flooded eastward across the midwest, behind the low exiting to the northeast of the great lakes. It has gotten ahead of any upper support and the layer of cold air in Ohio is very shallow, as seen on the ILN Skew-T plot from this morning (ILN is Wilmington Ohio, between Columbus and Cincinnati). So while it has pushed across the mountains, downsloping is in full force today, as can be seen from the satellite photo -- cloudy to the west of the mountains, clear here, and a northwest flow. Classic.
Fog has developed over the area as the front approaches from the west. The models picked this up to some extent yesterday; they were forecasting low clouds for our area this morning. Southerly flow has brought in some moisture from the Atlantic. With the low level moisture around, we could see some scattered showers as the front goes through this morning. But upper support is minimal.
The front brings in our first wave of colder air. The very cold arctic outbreak in the midwest is finally coming our way. Some upper support comes through later tomorrow in the form of some PVA and a broad upper short wave. This is easily seen on the 850mb, with winds shifting back to southwest overnight tonight, increasing RH tomorrow, and then a renewed push of cold air tomorrow night. 850mb temps drop below zero later tomorrow, and then to -10C on Friday. We could see some snow flakes as this cold air pushes in, but no accumulation is expected.
19 below zero Farenheit in Wyoming last hour! 9am Update: 25 Below in Wyoming!
The overall upper pattern over North America continues to support bringing air south from the Arctic into the middle of the CONUS, and the next shot of cold air is due to arrive there towards the weekend. The models continue to suggest the possibility of weekend cyclogenesis in the baroclinic zone along the gulf coast, but as yet, no forecasts of any big storms. With the cold air in place, any energy that comes up the eastern seaboard will create the potential for winter precip in our area.
At 7am in Cheyenne it was 50 degrees F. At 9am it was 18.
The cold air flooding south across Colorado:
Very nice weather here today and tomorrow and into Wed. Cold air is currently flooding south across the high plains, behind a low that is tracking across the midwest. A swath of heavy snow -- up to a foot -- is expected for parts of Minnesota and Wisconsin.
The leading edge of the cold air in the high plains is currently producing quite a strong temperature gradient. It is 70 in Colorado Springs, 30 in Fort Collins, and 16 in Cheyenne. The forecast for Denver is calling for the temperature to drop to 20F by 5pm. As can be seen on the plot below, the temperatures in Kansas are in the 70s and in Nebraska in the 20s.
That cold air will eventually get here, although modified quite a bit by that time. The last half of the week will be quite chilly here in the mid Atlantic states.
The cold air heads all the way to the gulf and sets up a nice baroclinic zone. Some sort of cyclogensis southwest of here seems likely for late in the week.
Gusty winds and wrap-around moisture from the intensifying low to the northeast is keeping the day pretty raw. Temps in the 30s to the west of the mountains and low 50s to the east show we are getting some downsloping, even if it doesn't feel like it.
A line of heavier showers moving through. Radar shows strong enough echos that there may be thunder to our south. So far I have not heard thunder nor seen reports of thunder.
The line is right along the leading edge of the vorticity max. We are also moving into the left front quadrant of a 300mb jet streak, giving a bit of extra lift (see the 21z 300mb map).
The steady rain has ended about when expected, based on yesterday's discussion: as the WAA ended, as seen on the 850mb charts. The 500mb PVA continues, so expect unsettled conditions until the vorticity max moves through this evening. The surface front associated with this vorticity maximum is back in the midwest and is foreast to move through our area this evening. Given the ongoing PVA, approach of the upper trough, and lots of low-level moisture hanging around, scattered showers are likely. SPC also suggests the possibility of thunder. The morning Dulles sounding is very saturated but also fairly stable. It will take some Cold Air Advection aloft (CAA) and some warming near the surface before any convection can take place. I'm a bit skeptical eough of this will happen to support thunder, so I'll watch the weather this afternoon with great interest to see what unfolds.
An upper system has moved by to the north and left a front over our area. Another upper trough is approaching from the west, and tonight we have a combination of PVA and WAA interacting with the front. This should give us a period of rain. PVA continues tomorrow, but the WAA ends in the morning. This should also end the rain. With the upper trough still pushing in tomorrow, there might be some scattered convection, although the moisture looks limited.
Below freezing this morning for many places along the eastern seaboard, including northern Florida. Temperatures in the immediate area are holding above freezing so far, with just a bit of wind. Places where the wind has calmed have dropped quickly to below freezing. It would be odd for northern Florida to have an earlier first freeze than northern Virginia!
As the upper ridge moves overhead and then to the east, our near-surface flow is forecast to turn to the southweast and bring in warmer temperatures. Today and tomorrow are looking to be very nice for early November. An upper trough approaches mid week. The models have not yet resolved the interactions between the northern and southern streams for this trough, so it is not yet clear how deep it will be by the time it gets here.
A very strong short wave diving very far south for this time of year. The 24Hr forecast is amazing! See below -- that is quite a small well defined low, very far south. It will draw some very cold air well into Florida, and drop some significant early snows in the Smokies. It's rotating off to the east and moving fast, so it looks like there won't be time for any significant moisture source to be tapped until the system has passed our area. So we may see some showers, and it also looks like we'll get our first freeze, if not tomorrow night, then Sunday night.
Look at the southeast US on the chart below. The cold air advection on this 48Hr GFS forecast is very impressive!
The models are showing a pool of cold air moving south from far northern Canada over the next few days. This cold air is moving fast! An upper short wave accompanies the cold air pool, and it is dropping almost due south and amplifying the long wave as it moves. The 72 Hour GFS forecast, for 2am Saturday, is showing a small closed upper low over eastern Kentucky, and a large cold high centered over western Wisconsin.
This is being forcast to be an unusually strong system, with highly anomalous 500mb height values for this time of year, and unusually strong gradients. Thus the confidence in the model forecast is low, especially beyond 3 days. This is a system I'll be keeping a close eye on.
Last night and today are a good example of radiation and the diurnal cycle. With a high overhead, the winds are calm, the sky is clear, and the dewpoints are low. The Dulles sounding this morning shows very stable air that is quite dry almost throughout the column.
So in the long autumn night, the temperature dropped steadily last night, from a high in the upper 60s at Dulles to a low in the mid 30s. With the sun, the temps have climbed very rapidly, already near 60.
A general northwest flow aloft continues, keeping our weather quiet. Shortwaves in the flow bring a few clouds from time to time, but for the most part it will be sunny. With a bit of downsloping we will se very nice daytime temperatures today and tomorrow. A high building overhead, then moving east, will give us some low level southerly flow early next week, with warmer temperatures.
Long term, the models are suggesting a big cold outbreak around Halloween. Too far out to have much confidence in it.
As the surface low strengthens to our east, pressure gradients will tighten so it should be breezy today. The upper low overhead along with lower level circulation from the ocean is keeping a band of precip over our area. It appears the western edge of the precip is currently near the beltway. This will be slow to move out.
The circulation around the surface low is also bringing cold air from the north, so today is the wet, raw, and dreary day we were expecting. As the upper low slowly moves out, we should see some sun tomorrow, although it may not clear out until later in the day. Another upper short wave moves through Friday, so right now expecting some sun and some clouds.
The latest 7 day GFS forecast shows a very powerful looking storm approaching the pacific northwest! Sort of looking like a hurricane? Oh well, it is a 7 day forecast....
The high thin cirrus did hold in overnight; they were quite visible at sunrise. It does appear most places avoided frost and freeze last night.
Another upper trough approaches today, so we will see increasing clouds due to the lift ahead of the trough. The models show the trough digging and closing off just to our east. With an upper low overhead and the associated cyclonic flow, we'll see clouds and a risk of showers for a couple of days, until the low lifts out of our area. The models bring a lot of cold air with the low, so it looks like the combination of clouds, cold, and wet will make things downright dreary around here for Tue and Wed.
Watching the sunset, I see a lot of high clouds around. Looking at the model forecast for 300mb relative humidity, I see the GFS is forecasting high RH at 300mb overnight. So the high thin cirrostratus layer is likely to persist overnight. This may lessen net outgoing longwave radiation just enough to keep us from a freeze tonight.
Tonight we will see high pressure overhead giving us clear skies, low dewpoints, and calm winds. A long autumn night with ideal conditions for radiational cooling. Dewpoints are running in the upper 20s to near 30. For me, there is sufficient risk of frost tonight that I am picking the last of my fall crops.
Cold air advection visible all day at 850mb and the 1000-500mb thickness, but still a fairly nice day here with the downsloping. But now the next shortwave is approaching, plainly seen on the 500mb chart. It is bringing some clouds, and yet more cold air advection. This is starting to dominate over the downsloping and giving us dropping temps. Tomorrow behind the shortwave it will clear out, and be chilly as high pressure builds over the area. Keeping a close eye on any frost and freeze risk for tomorrow night.
Longer term, the models are still closing an upper low over our area on Tue/Wed, giving us a risk of rain and clouds and keeping us on the cool side for this time of year.
Nice day today. Over the weekend the overall pattern is one of cold air advection (and drier air) over most of the eastern half of the US. There will be a couple of strong shortwaves rotating through the northeast, and with each of them there may be some clouds, but with limited moisture it is unlikely we'll see any showers.
As usual for this time of year, it is important to keep an eye out for any frost or freeze potential. The first freeze is an important event for many people. For example, I have about 10 bell pepper plants that are all loaded with peppers. I am leaving the peppers on the plants as long as possible, to get as many red ones as possible, and for the green ones to mature. The day before the first frost or freeze, I want to pick everything. Even a light frost will damage the plants and fruit a lot. I don't want to lose the 2 or 3 bushels of peppers that are currently on the plants!
Also, I still have late season apples on some of my apple trees. These will not be affected by a frost, but they will be damaged by a freeze. Again, I am leaving them on the trees to ripen as much as possible. Cold nights are good to have as they sweeten the fruit. But I want to pick them before a first freeze. I also want to pick them if there is going to be a strong wind event, as that will knock the apples to the ground and bruise them.
Longer term, the models are closing off another upper low right over us. The details of this are not yet clear, but it does look like an overall cold and cloudy pattern next week with a risk of rain.
Classic comma head. Mature closed-off vertically-stacked low.
A bit drier and sunnier than I expected yesterday, but highs were still limited to around 80 or so. Still warm overnight, but the front is just to our west and rain is moving in. Temperatures at Winchester dropped 10 degrees in one hour when the front passed there.
The band of heavy rain with the front is pushing slowly eastward, as cells within the band move south to north. This creates a potential for heavy rain and flooding, particularly given the moisture feed visible in the models. Mesoscale effects will determine the locations that get the heaviest rains.
The upper low is overhead tomorrow, so expect continued cloudy with showers in the area. The upper low finally moves out tomorrow night, so Friday should be a nice day.
As the large closed upper low opens up and moves towards the northeast, it intensifies the long wave over eastern Canada. The large scale circulation then brings an air mass down from central Canada. The strong front can easily be seen on the three day animation of the 850mb level, pushing across the upper midwest. This cold air is forecast to arrive here over the weekend. So longer term, we can expect much cooler temperatures.
Temperatures have been steadily increasing as the cold air erodes. Dewpoints have also been increasing. Very warm and humid today, with temperatures in the 70s. If the sun is able to break out for any length of time, we may even get close to 80. I expect that any sun will quickly generate showers.
The big closed off upper system to our west will slowly move east, and bring us increasing chances of rain. Viewing an animation of precipitiable water shows two streams of moisture being drawn into our area ahead of this system -- one from the gulf, and another from the Atlantic. As they merge over us tomorrow, our pwat (precipitable water) values get close to 2 inches. This moisture feed, along with the organized lift ahead of the upper system, may bring us heavy rains tomorrow. The situation is likely to be complex, with various mesoscale and terrain interactions. The models are poor at resolving these details.
Chilly, dreary, drizzle, showers -- typical cold air damming situation. Will the cold air erode ahead of the next system? This is something the models do not handle very well. The big trough in the midsection of the country is closing off and slowing down, and the models are not handling this well either. So there is a fair amount of uncertainty in the forecast.
Today I'd expect continued dreary weather, and tomorrow may be the same. But given the time of year, the cold air may erode and give us a warmer and drier day.
It is worth looking at the 12Z Dulles sounding today. A stable inversion up to about 850mb, and the sounding saturated, or close to saturation, through almost the entire troposphere.
It is also worth the time to look closely at the strong trough at 500mb which is digging well south and then closing off. Look especially at the 300mb jet streaks and the location of the surface low pressure. It is a great opportunity to get a sense of how such systems evolve both in time and three-dimensionally.
We didn't get much clearing, but it cleared out to the north of us. The clouds are now back in, and there are a few light showers around. As the trough in the middle of the US strengthens, we will be in southerly flow, with a lot of warm air advection and moisture advection from the gulf and Atlantic. The cold air currently in place will take a while to erode, but I think it likely that will happen by Tuesday, making Tuesday a warm and breezy day.
The models have not been doing very well with the trough digging into the middle of the US. The latest runs show the trough closing off and moving more slowly in our direction. Looking at the NCEP Model Diagnostic discussion, it sounds like other models around the world are even slower with this trough. So it is now looking as though the trough and associated frontal boundary won't clear our area until Wed or even later.
So with the cold air damming, I'd expect to see continued cool and cloudy conditions tomorrow (Mon) with a risk of a shower at any time. But it should be mostly dry. Then Tuesday if we erode out the cold air, we should see breezy and warmer, again with the risk of a shower at any time. Might even see a bit of sun. If some sort of mesoscale thing gets going, we may get a period of heavier rain on Mon or Tue, but right now that does not appear likely. Then Wed there is the potential for heavy rain with the approach and passage of the main trough and frontal zone.
Still some clouds and rain around, but the rain should be moving out soon. But with all the low level moisture in place, it will stay cloudy and perhaps some drizzle. The high to the north pushes in towards tonight, so some drier air should work into the area. There may be some clearing, but the odds are greater the further north one goes. Frost advisories have been issued for south central PA, where it is expected to clear out overnight.
Tomomrrow we may see some sun, and then early next week the flow turns southerly and strengthens ahead of the powerful system in the midwest. The cooler/colder air will be well entrenched over our area near the surface, and it will be difficult to disoldge. But the low level jet ahead of this next system looks impressive -- 70 knots at 850mb over Indiana late Monday!
With that much southerly flow along with warm air and moist air advection, the weather ahead of the next system will be unsettled with a risk of a shower at any time. But the heavier rain comes in on Tuesday, and right now it looks to be a significant event.
The forecast looks on track for today. Cyclonic flow aloft and a strong overrunning situation will give us periods of rain for today and tonight. High pressure at the surface pushes in tomorrow, but continued cyclonic flow aloft and various small short waves may keep us mostly in the clouds and continue a risk of rain for the weekend. One of the shortwaves in the polar jet seems a bit stronger and moves east of our area late Saturday, which may clear us out for a while on Saturday night and maybe even on Sunday. The cold high moving in will keep temps chilly.
The models are still showing a deep high-amplitude trough forming in the middle of the US early next week. The trough digs way south, and the southerly flow ahead of this trough taps gulf moisture for a prolonged period. This much moisture gives a heavy rain threat to many areas east of the Mississippi. Given the long lead time, there is a lot of uncertainty. For us, the heaviest rain threat appears to be for Tuesday.
Some clouds already overhead this morning due to the developing stationary front and the beginnings of overrunning. The models are moving a wave along this front giving us rain tomorrow. I do not see any significant jet dynamics or vorticity advection, so the lift must be primarily warm air advection. The 850mb level shows good warm air advection ahead and with this system. I'm not seeing heavy rain due to the lack of other factors.
The models show high pressure building into our area for Saturday. It seems plausible that the cold air over the northern midwest this morning will want to move. It is not obvious as yet if this air mass will be strong enough to clear us out for Saturday.
It looks like we may get a warm front approaching later on Sunday, so probably back to the clouds (if it ever even clears out). But the uncertainty for Sunday and beyond is high, given the overall pattern. Remember that on Tuesday I mentioned the models forecasting a big jet to crash into the pacific northwest late in the week. This is still forecast to occur, and the jet max crossing the Rockies (see Sunday's forecast for the 300mb level) will predictably form a low on the lee side -- in this case, the low is forecast to form over Oklahoma.
The upper jet diving towards the southeast over the Rockies on Sunday is forecast to dig a big trough over the central US by early next week. The energy for all this is still over the Pacific (a data poor region) so the details certainly are unclear this far out.
But this should be an interesting system to watch as the situation unfolds.
The satellite shows very nicely the big "vertically stacked" low that is centered near Hudson Bay. We are seeing some clouds here, even this far south, from a combination of surface heating and cyclonic flow aloft.
A nice day today with surface high pressure building in and a bit of downsloping with the westerly flow at low to mid levels. But the long wave trough is still centered to our north keeping us in cyclonic flow. The models over the next 48 hours show a strong front building overhead, streatching from our area to the west around the Kansas and Oklahoma area. This can be seen very well on the 850mb charts, with nice isotherm packing, wind shift line, and high relative humidities withe the lift from overrunning. The flow south of the front is from the south off the gulf, so there will be lots of moisture to overrun the front. This front looks to stay near us for several days, so expect clouds to return tomorrow, followed by unsettled weather for Friday into the weekend, with a good chance that we will see periods of precip.
Very clear this morning for the eclipse.
A short wave rotating around the long wave trough will bring us some late day showers. The models are showing this short wave moving out of our area after midnight, with some NVA (negative vorticity advection) towards morning. The models also show low RH values at 850mb towards morning. I am getting optimistic the lunar eclipse may be visible from our area.
Tomorrow looks like quite a nice day. The models show frontogensis and a stationary front over our area, which may bring clouds on Thursday and then precip for Friday. The weekend looks unsellted overall, but the details are too hard to pin down at this point.
It stayed pretty warm overnight but the front is now pushing through the area. Dewpoints are starting to drop at Winchester, and will be dropping here in the next few hours.
The SLP/1000-500mb-thickness chart shows very strong CAA for the next 12 hours and moderate CAA continuing overnight. We'd see falling temperatures all day based on those maps, but the strong westerly winds will result in downsloping here, and that will keep the temperatures steady today until later in the afternoon, when the temps will start to drop very quickly.
It should get quite breezy later today with the strong pressure gradient. In addition, the combination of strong CAA, sunshine, and strong winds aloft will enhance the windy conditions (this combination allows the winds aloft to mix down to the surface).
There is a very strong vorticity max rotating over our area today, which may give us some clouds. But the cold air aloft, as seen by looking at the 500mb temperature chart, does not arrive until overnight. This will keep us more stable today, and that combined with the downsloping, may act to minimize the amount of clouds.
The charts show WAA returning on Monday, so our cold blast will be short lived.
There's been nothing to move the low level moisture out of our area, so again last night we saw low clouds and fog. There's even a band of precip just to our west at this time. Perhaps a bit of a warm front aloft?
We've got big changes coming. The lead shortwave of the big system in the mid section of the country is moving east rapidly and weakening. It is bringing the surface front with it, but the main longwave, with imbedded short waves, is still over the midwest, and does not fully clear our area until tomorrow night.
So as the front approaches and the pressure gradients intensify, we'll see some increasing winds -- from the south today, then more westerly tomorrow behind the front. We'll probably see a bit of downsloping tomorrow, which will help to suppress the potential for clouds and showers from the vorticity max aloft. But with it so cold aloft, and with the strong upper support, I'm still expecting afternoon clouds and perhaps some showers. Tomorrow night it gets quite chilly!
In the long range, the GFS is picking up a tropical system west of Baja and bringing it northeast into the desert southwest. It then continues to move it towards the north and east and builds a trough to the lee of the Rockies. This is really interesting! It will be something to watch, to see how the model performs with this situation.
Lots of low level moisture present, as shown by the Dulles sounding this morning -- near saturation up to about 800mb. Radiative cooling overnight resulted in a lot of low clouds and fog. There's not much wind to change this situation, but daytime heating should clear out some of the clouds as the day goes on.
The overall pattern shows ridging aloft and at the surface, so a quiet day here. Out in the midwest, a strong short wave trough is rotating around the long wave that covers most of weastern N. America. This potent short wave trough is moving through the southern plains and gulf states. SPC has issued a large slight risk area where the trough interacts with sufficient low level warmth and moisture.
This initial short wave trough gets absorbed into the long wave trough tomorrow, but a 2nd short wave is right behind it, and that short wave has a powerfull jet associated with it. This drives south and amplifies the long wave trough, and the whole thing moves through our area late Friday and Saturday.
The models forecast our 850mb temperature to dive from 14C late Friday to -2C late Saturday. If this verifies, it will be our first sub-freezing temps at 850mb for the autumn.
The very potent long wave trough is forecast to have some complex imbedded short waves as it moves through our area, which makes the forecast for Saturday problematic. The models are not currently showing any precip for Saturday, but with such strong dynamics aloft, I am skeptical at this time. In addition, the forcast low level winds (eg, 850mb) do not seem strong enough to me for such a strong system aloft.
A small blob of light rain right over us! The front that was over Ohio yesterday has weakened but made it across the mountains into our area. This combined with easterly moist flow off the ocean has set up a bit of a convergence zone over our immediate area. Then there is the upper trough/low moving through to provide a bit of lift. So a bit of rain is not surprising -- buy why is it just over us? I guess we are just unlucky.
As the upper low moves east, conditions should improve through the day.
SPC has expanded the areal coverange and the level of risk for tomorrow's outlook. Something to keep an eye on.
The models have increased the strength of the trough moving through our area for Fri/Sat and are bringing more cold air south from Canada (as compared to yesterday morning's model runs). Sterling's discussion mentions the risk of a QLCS with the front (Quasi Linear Convective System).
Looks like a big pattern change in the long term, with the weekend system closing off over the north central CONUS and drifting east into next week. It will bring weather that will feel cold to some people. We'll have to keep an eye out for any risk of a frost or freeze. Not too likely as it appears now, but...