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8am 09 Dec, 2010:

Very cold temperatures this morning. Note that just to our south and southwest, it is 10 degrees colder than in our area. Looking closely at the station models, it can be seen that some wind is being reported in our area, so conditions here did not go completely calm. However, to the south and southwest of our area, winds did go completely calm at many locations, which has allowed the temperatures near the surface to drop further.

With the surface high pressure moving overhead during the day today, conditions will remain clear and cold, although the winds should diminish. This will also give us another cold night tonight -- perhaps even colder than last night, since winds may go completely calm over our area. As can be seen by the forecast map below, we get warm air advection tomorrow as the high moves to the east.

8am 06 Dec, 2010:

The clipper system ("clipper" being the term used for a typically winter system that moves from northwest to southeast in fast flow) that moved to our south on Saturday went on to the east to rapidly form an intense low over the Gulf Stream which has wrapped to the north and intensified to a 970mb low over Nova Scotia. This has strenghtened the pressure gradient over us giving us persistent northwest flow extending from nothern Canada. The forecast is for a huge vertically stacked low to form over southeast Canada keeping us in this flow regime for at least a few more days. A weakening of this low is forecast by later in the week, with some warm air advection. Longer term the models suggest the strong eastern trough/ western ridge pattern will reform, with strong east coast cyclogenisis. It is far to early to look at details of this possible storm, but the HPC extended range forecast discussion will be interesting to follow.

1pm 01 Dec, 2010:

The frontal passage at Dulles is very distinct.

9:30am 01 Dec, 2010:

Not too surprisingly, a tornado warning has been issued for southern PG county and northern Charles county, associated with the LEWP feature. A close look at the 9am surface anlaysis shows a cold front moving towards the metro area from the northwest, and a dry line moving towards the area from the southwest. Ahead of the front the winds are from the south or southeast. Note that behind the front, there are strong NW winds at Dulles, Frederick, and Winchester, while the winds are from the SW at Warrenton and Charlotsville. The LEWP appears to be at the intersection of these fronts, and the intersection is moving almost due east, where the tornado warning has been issued. The dry slot to the southwest can also be seen on the satellite photo.

9:15am 01 Dec, 2010:

Dulles reports a wind shift at 8:35am which probably marks the frontal passage there. The temperature at Dulles is 45F at 9am, down from 63F at 8am -- an 18 degree drop in one hour. Here in Falls Church it appears the front is currently pushing through, with very heavy rains, a marked temperature drop, and high winds -- I estimate gusts between 40 and 50mph. Radar shows a line of heavy precip right along the front moving through the metro area. Note the LEWP (Line Echo Wave Pattern) just to the SW of the beltway, indicative of mesoscale circulation and strong dynamics.

8:30am 01 Dec, 2010:

The movement of the front seems to have slowed considerably for the last few hours, but now is on the move again to the east. Temperatures at Winchester VA have fallen to 44 degrees at 8am, from 62 degrees at 5am. At 8am the front appears to be right over the blue ridge mountains. The tornado watch is still in affect for our area. A number of stations in WV and western PA are reporting snow.

6am 01 Dec, 2010:

Winds gusted to 41 knots (47mph) in the last hour at Dulles. Overnight, wind gusts over 60mph have been reported at higher elevations in the mountains.

5:30am 01 Dec, 2010:

Oops, just had a power blip. Current radar shows the front just to our west with a strong cell near Warrenton. NWS has issued a severe thunderstorm warning for this cell. There are also flash flood warnings issued for many counties to our west. Maximum pressure falls in the last hour are right over our area, with a dramatic pressure fall/rise couplet.

5am 01 Dec, 2010:

Looking out the window this morning at 430am, I saw a bright green flash in the distance, indicating a power line arc (green from copper). Strong winds are mixing down to the surface ahead of the cold front. Looking below at the 4am surface analysis, we can see that the very strong upper dynamics has formed a low on the front, currently just west of Charlotsville. As this low tracks to the northeast, it will likely be the focus for any severe weather that may form. SPC has issued a Tornado Watch for our area valid until 10am. They say in their discussion (with abbreviations expanded):

"Low-topped convection now present near the central Virginia surface low is expected to move/develop rapidly NNEwd in an environment of very weak buoyancy but very strong low level shear. The main severe threat likely will be isolated damaging wind from small bows ... but a short-lived tornado or two cannot be ruled out ... mainly near/E of the surface low."

Although the heaviest rain will be with the front, the 500mb vorticity maximum does not reach our area until midday, so the precipitation may not completely end until then. Temperatures are expected to fall rapidly this afternoon behind the front, with the strong cold air advection overwhelming any downsloping effect. Note on the forecast map below, for midday today, the packing of both the isobars and the thickness lines over our area.

9pm 30 Nov, 2010:

Current surface SLP analysis shows the front clearly as an elongated trough of low pressure. Temperatures to the east of the front are generally in the 50s and 60s with dewpoints in the 50s. To the west of the front, temperatures are in the 30s with some locations in Ohio reporting snow. Note the strong winds to the east of the front, uniformly from the southeast, feeding moisture from the Gulf and Atlantic. As the upper trough is now taking on a more negative tilt, resulting in increased height falls and PVA, the surface convergence is increasing. The radar shows a well defined band of heavier rain right along the front, stretching from far southeast VA almost due north across WV. This band suggests the front may be starting to interact with the warmer and more moist air to the east of the mountains. Will a line of thunderstorms form along the front, in spite of the time of day and the time of year? This is still a possibility, and SPC still is showing the area just to the south and east of Washington in a slight-risk area for severe thunderstorms.

5pm 30 Nov, 2010:

Today's model forecasts for overnight tonight are very similar to yesterday's runs. They produce a heavy swath of 2+ inch rains over a large part of the eastern seaboard. Although the models are forecasting the heaviest rains to be over the mountains to our west, some heavy rains for our area are still likely. The heaviest rains are forecast for the early morning hours on Wed. and possibly into the morning rush hour. The rain overall may not end until midday. SPC has us included in a slight risk area for severe thunderstorms -- if any thunderstorms do form, the very strong winds just a few thousand feet overhead will be easily transfered to the surface. A flash flood watch and a wind advisory have also been issued for our area.

8am 30 Nov, 2010:

Overall the forecast for heavy rain with the overnight frontal passage looks on track. I will try to post a full analysis this afternoon after the 12Z model runs are available.

This morning we are seeing some light rain from the warm air advection ahead of the upper system overrunning the surface warm front. Looking at the current surface wind analysis, the low over the western great lakes is clearly visible, as is the trailing cold front extending south through Indiana then to Alabama and the gulf. A diffuse warm front can also be seen over North Carolina, which can also be seen on the temperature and dewpoint analyses (not shown). Further below is the SLP change over three hours. The broad scale pressure falls over the entire eastern US show the large scale warm air advection over this region. Given the upper jet structure, there is no strong cyclogenesis at this time (except with the primary surface low moving into Canada), so there is no focused area of pressure falls. This may change later today as the upper shortwave begins to rotate around the base of the larger trough and causes the trough to go negative tilt.

3pm 29 Nov, 2010:

I hope everyone had a nice Thanksgiving. I did not post during the break, but now that we are back, the weather situation is certainly quite interesting. A deep upper trough is currently over the southweast US. Imbedded within the large trough can be seen two shortwaves, one over the central plains, with another over sourthern AZ/NM.

The 300mb jet shows an exit region over the central midwest, with the entrance region currently to the west of the trough over ID and UT.

The current surface analysis of SLP shows a primary low over the central midwest associated with the leading shortwave at 500mb and the 300mb jet exit region. The low pressure trough extending north/south from the low marks the frontal boundary, also easily seen on the current maps of temperature, dewpoint, and winds (not shown).

Also of note is the high pressure over the NE US with a ridge of high pressure south along the eastern coastal plain. Note the north/south orientation of the isobars over much of the central US, indicating a very large area of southerly flow setting up, feeding from the GOM (Gulf of Mexico).

Looking at the current map of precipitable water, moisture over the southern states can be seen, with a large area of values over 25mm (appx. 1 inch).

Looking now at the forecast maps for early Wed, the lead shortwave has moved towards the north, as has the exit region of the jet. This results in the primary surface low moving through the western Great Lakes and then further northward towards Hudson Bay. By tomorrow evening the 2nd shortwave is shown rotating through the base of the trough, resulting in the trough going negative tilt, with extremely impressive height falls and PVA over our region.

As can be seen from the 850mb forecast for about the same time, the trailing cold front extends from the primary low southward along the Appalachians. The potent upper shortwave rotating into our region interacting with the strong cold front moving towards us from the west -- this is expected to be a significant weather maker for our area. Also note the very strong low level southerly jet ahead of the front, pulling in warm moist air. Looking at the other levels (not shown), a very strong upper jet will be over our area as well. With strong vertical motions and jets, and with ample available moisture, heavy rain is expected in our area as a wave forms along the front due to the strong upper shortwave rotating through our area. Flash flood watches have already been issued for our area for later tomorrow and overnight tomorrow.

Even though instability is forecast to be weak, the possibility of thunderstorms cannot be ruled out, given the strong dynamics and available moisture. Even a small amount of convection would mix the strong winds aloft down to the surface, which gives our area a risk of strong wind gusts in addition to the heavy rains.