324 fxus61 klwx 211051 afdlwx Area forecast discussion National Weather Service Baltimore Maryland/Washington District of Columbia 551 am EST Sat Nov 21 2009 Synopsis... high pressure will cross the region and settle over New England through the weekend. A low pressure area will move up coastal Carolina Sunday night into Tuesday...bringing rain. High pressure will return late Tuesday...though a cold frontal passage is expected in the middle to late portions of next week. && Near term /until 6 PM this evening/... a quick update for sky cover early this morning. Stratus deck around 4 kft has made it east of the Blue Ridge mountains. Will go ahead and bump up the sky cover between the Washington metropolitan area and the Mason Dixon line. Otherwise...no changes. Previous discussion below. High pressure will remain across the eastern Ohio Valley today. Light northerly surface flow will continue...with highs ranging from the middle 40s in the higher terrain to upper 50s across the Piedmont. No precipitation is anticipated. Water vapor loop and 500mb analysis shows a trough across the central Great Lakes...with strong shortwave energy rotating through the base. The trough will continue to move eastward today...and set up along the Mason Dixon line through tonight. The GFS and NAM are slightly different with their solutions for the trough evolution...and both initialized rather well. The NAM develops a cut off low across the northern County warning forecast area overnight...while the GFS maintains a shear axis. Neither model is producing precipitation...just increased cloudiness. 250mb jet streak continues to help spread cirrus across the County warning forecast area early this morning. Expect the cirrus to hang around most of today...at times becoming opaque. Have opted to go partly sunny for today...rather than mostly sunny for most of the County warning forecast area. The northwest County warning forecast area should remain cloudy/mostly cloudy in the broken/overcast stratus deck. Thicker cirrus could lend to temperatures being a degree or so cooler than guidance. Since the mav MOS has been performing rather well...have used those numbers and bumped temperatures down a degree or two. && Short term /6 PM this evening through 6 am Sunday/... cirrus and lower stratus deck in the northwest are expected to thin out a bit by the evening...however moisture from the Gulf states system is expected to stream northward overnight. Partly cloudy/mostly cloudy skies are expected mainly after midnight. Mav numbers continue to verify slightly better than the other guidance...and have used for the overnight mins. && Long term /Sunday through Friday/... Sunday...flow becomes northeasterly with high pressure over New England and an approaching coastal low off the Carolinas. Clouds in the region due to coastal system...so expect mostly sunny skies until cloud bank rolls north-Ward in advance of the low. With more sun northern zones...maximum temperatures Sunday could be the inverse of normal with warmest /around 60f/ locales in Harford/Baltimore and cooler /mid 50s/ Nelson/Albemarle. Sunday night through Monday night...high pushes off New England and low pushes east of Hatteras...bringing Ely flow/cad to the area. Having already moistened...expect light rain spreading east across the area with east facing slopes having the greatest precipitation potential due to topography. Should be quite cloudy with some rain...expect low diurnal range...mins middle 40s...maxes middle 50s. Tuesday and Wednesday...a closed low over the Central Plains/middle west to break cad with southwesterly flow by Tuesday afternoon /did increase probability of precipitation for Tuesday morning across eastern half of County Warning Area due to extended cad/. Warming trend Tuesday /upper 50s/ to Wednesday /around 60f/ ahead of frontal passage. Wednesday night/Thanksgiving...short wave lobe/S/ from the low to bring one or more cold fronts passages in middle to late week timeframe bringing precipitation chances across the County Warning Area. However...moisture availability for the middle Atlantic is uncertain. Cold air does look to follow the front...bringing a cool down and mountain wintry precipitation. Thursday night/Friday...models uncertain of low intensity and track...but the potential is there for an early upslope snow event in northwesterly flow around the backside of the low feeding Great Lake moisture to the Allegheny Front and possibly ridges further east. Will have to monitor future models for consistency/greater discernment. If anything...cooling trend late next week. && Aviation /12z Saturday through Wednesday/... VFR conditions continue across the County warning forecast area as high pressure remains across the eastern Ohio Valley. Upper jet streak continues to help spread cirrus across the area tonight...with ceilings well above 3 kft. Some lower stratus has settled across kmrb...in the northerly surface flow...but ceilings are around 4 kft. Expect the cirrus ceilings to continue through the afternoon...at times broken. The lower stratus should continue to slowly spread along the Mason Dixon line...but ceilings below 3 kft are not expected. Onshore northeasterly flow sets up Sunday...with moistening/precipitation beginning by late Sunday night as Ely flow sets up across the terminals. Low ceilings and reduced visibilities in rain/drizzle into Tuesday morning. Southwesterly flow ahead of the next cold front later Tuesday/Wednesday with improved aviation conditions. Quantity of precipitation associated with front looks to be low at this time though with timing later Wednesday/early Thursday certainly pay attention to subsequent forecasts. && Marine... northerly flow is expected to continue across the marine area thought tonight...with wind speeds 15 knots or less. High pressure building over New England and a coastal low moving up the Carolinas Sunday/night brings about northeasterly flow. Small Craft Advisory winds possible Sunday night through Monday night. Southwesterly flow ahead of the next front later Tuesday and Wednesday. One or more fropas later Wednesday through Friday to bring some precipitation and colder air. && Lwx watches/warnings/advisories... District of Columbia...none. Maryland...none. Virginia...none. WV...none. Marine...none. && $$ Update...listemaa