... An upper level disturbance will linger over the region today. It will move away tonight. A strong cold front will cross the region on Saturday. Canadian high pressure will then build across the region Sunday through early next week.
NEAR TERM /THROUGH TONIGHT/...
Low amplitude trough over the OH Valley will progress east crossing the area late this afternoon and exiting the Chesapeake Bay by 06Z tonight if not sooner. As this trough interacts with moisture laden atmosphere, showers and thunderstorms, some with very heavy rainfall, will develop. Environment today is a lot more conducive to flash flooding with half inch precipitable water values higher than 24 hrs ago, slower 0-6km mean storm motions and MBE velocity vectors, high K indices, and no dry air present in the 12Z IAD sounding. Timing looks to be earlier today by a couple of hours and earlier ending time. Flash Flood Watch was expanded northwest to add Washington Co. and more WV counties.
Late tonight, the trough passes east, and we should become very quiet (drier and quieter than this current overnight has been). However, it will remain soupy, with lows in the 60s to near 70.
SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/... Canadian cold front pushes southeast across the region on Saturday, followed by cooler and drier air associated with Canadian high pressure on Sunday. Upper level forcing lags behidn the front on Saturday and westerly flow should develop ahead of it. Thus, we are not impressed with convective coverage Saturday, but some storms may develop later in the day, most likely south of the DC metro area. These storms could pose a severe risk, but should be moving right along, so the flooding risk looks reduced. Highs Saturday should be a little warmer than today with greater sunshine and westerly downslope flow, but not much.
Saturday night through Sunday night, the Canadian high pressure builds in, with much drier and somewhat cooler air. Lows Saturday night will be in the 50s and low 60s, with 50s almost everywhere on Sunday night. Highs Sunday will be near 80, and dew points will be much lower, 25-30 degrees lower than today.
LONG TERM /MONDAY THROUGH THURSDAY/... Medium range guidance remains in very good agreement with synoptic scale features through much of the long term period, though there is still uncertainty surrounding the details of an approaching trough by the middle of next week.
Deep layer ridging will build over region from the west Monday into Tuesday of next week. leading to a warming trend of temperatures. As the ridge axis moves offshore, return flow will lead to an increase in humidity and continued warming temperatures during the middle of next week.
A large upper trough will pivot across the Plains and Midwest by the middle of next week, absorbing part or all of the remnants of Cristobal before heading generally northeastward. Precip chances will increase by next Wednesday-Thursday, but the magnitude will depend on finer scale/track details still not readily apparent at this time range.