550 fxus61 klwx 211943 afdlwx Area forecast discussion National Weather Service Baltimore Maryland/Washington District of Columbia 243 PM EST Sat Nov 21 2009 Synopsis... high pressure will remain over the region tonight before settling over New England Sunday. An area of low pressure will develop off the middle-Atlantic coast Monday before slowly moving up the eastern Seaboard Monday night into Tuesday...bringing rain. High pressure will briefly build in late Tuesday into Wednesday...but another cold front will pass through late in the week. && Near term /through tonight/... current analysis shows an area of high pressure over the Ohio Valley that extends into the mid-Atlantic. A split jet continues across the central and eastern Continental U.S.. an upper-level low in the northern branch is moving into Pennsylvania this afternoon while an upper-level low in the southern branch tracks into the Gulf Coast states. Surface high pressure will continue to bring dry and seasonable conditions tonight. The strato cumulus deck associated with the upper-level low over PA should dissipate across most areas this evening with the loss of daytime heating. However...clouds may hang on across the northwestern County Warning Area into the Potomac Highlands where there is a little more low-level moisture. Min temperatures will range from the lower and middle 30s in portions of the Potomac Highlands into the Shenandoah Valley...to the middle 40s in downtown Washington/Baltimore. && Short term /Sunday/... the upper-level low currently located over Pennsylvania will slowly track south into our region while the surface high builds toward New England Sunday. The southern branch system will track into the southeastern Continental U.S....but it should remain far enough to our south not to have any impact on our weather through Sunday. A low-level north-northeast flow around high pressure to our north will keep dry conditions in place along with sunshine. Maximum temperatures will top off in the middle to upper 50s across most locations...but upper 40s in the higher elevations. && Long term /Sunday night through Saturday/... guidance trending much sloer...holding off precipitation Sun night as strong hipres over new englnd rdgs down Atlantic CST. Emphasis of rafl now Monday-Monday night...as a weak WV in the southern stream spills moisture northward...and eventually spawns coastal low. Warm air advection on southern doorstep sunset sun...and clouds will thicken/lower through evening. Will bring probability of precipitation into grids lt sun evening/sun overnight...especially southern counties which is nearer triggering mechanism. By Monday-Monday night...energy will Transfer off CST. Precipitation will be in response to developing low...mostly in deformation zn. Have lingered a chance into Tuesday...especially northern counties. However...cold air advection should be able to mix out the deep moisture. But...flow wont go westerly...and as such am afraid will hold onto a prtl marine deck into midwk. Per extended dscn 2 paragraphs down...forecast at this pt becomes a bit more uncertain...and gridded details become broader in scope. Per temperatures...have limited diurnal swings in response to cldcvr next cpl days...going on high side for min-T...and low end for maxt. Did shade dwnwd Monday night since County warning forecast area will be affected by cold air advection by then. We/ve entered the time of year when split flow wrecks havoc on model solutions. It appears we/re entering such a period. The main forecast challenges revolve around shortwave energy digging into trough across the plains...likely cutting off an upper low...and then traversing the Continental U.S. As addtl pieces of shortwave energy dive into the trough axis. GFS and ggem deeper and quicker west/ 500 mb low...which is almost contradictory. Will take a highly ensembled approach...leaning on steadier European model (ecmwf). Regardless... will have an upper trough in the neighborhood /prbly centered over the Great Lakes/ this thxgvg...which means the extended forecast will start out warm...then have a cold front passage cool County warning forecast area off. There also will be the support for precipitation-- rain at 1st...but heights/tknss/h8 temperatures would support at least some snow...especially for the mountains by Thursday night..which may continue into Friday across western upslope areas. Pop forecast low confidence at this time...but will continue to be monitored due to potl trvl impacts. The western upslope aspect to the forecast the most solid. && Aviation /20z Saturday through Thursday/... patchy br may bring a brief period of MVFR visibilities early Sunday morning...otherwise VFR conditions are expected tonight through Sunday. Light northerly winds are expected tonight...winds should remain below 10 knots Sunday with the directions veering toward the northeast. VFR sun evening...as clouds thicken/lower from S-N. Flgt restrictions possible overnight as rain spreads in. Restrictions prevalent Monday-Monday night as lopres passes east of terminals. Should be a break Tue-Wed...though marine layer could potentially hold on. Then...anthr storm system mvs in Wednesday ngt-Thu...fllwd by cold air by end of weak. && Marine... high pressure over the waters tonight will build toward the north Sunday. Northerly winds tonight will veer toward the northeast during the day Sunday. Wind speeds will remain below Small Craft Advisory criteria. Weak WV of lopres tracks across southeastern Continental U.S. Sun night...and transfers/deepens off Delaware-Maryland-Virginia Monday-Monday night. Winds increase during this period...W/ Small Craft Advisory conds possible. In wake of this low and in advc of next storm systm...north winds veer Ely. More precipitation will spread in midwk...but Small Craft Advisory not expeceted at this time. && Lwx watches/warnings/advisories... District of Columbia...none. Maryland...none. Virginia...none. WV...none. Marine...none. && $$ Synopsis...bjl near term...bjl short term...bjl long term...heights aviation...bjl/hts marine...bjl/hts