Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 221906 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 306 PM EDT Fri Jul 22 2016 .SYNOPSIS...
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High pressure will remain offshore tonight, as a weak trough remains over the Mid-Atlantic. A weak front will arrive from the north on Saturday. High pressure will then be in control of the area`s weather Sunday before another front enters the area Monday into Tuesday. This front may stall across the region during the middle of next week.
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&& .NEAR TERM /THROUGH TONIGHT/...
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Synoptic features are rather subtle over the Mid-Atlantic with high pressure offshore and a weak front well to the north. Large upper ridge over the Plains will be a predominant feature through the weekend. An early morning MCS from Lake Erie finally decayed over SW PA, although new convection is developing on its forward flank. Think the best chance for some showers and storms this afternoon will be over the western mountains. The dense remnant cirrus has helped limit the cumulus field (in addition to the temperatures). However, as the subtle boundary drifts south into the warm and humid airmass late this afternoon and evening, there`s a possibility there could be a few storms farther east as well. Gusty winds are possible with high DCAPE values. Any activity should diminish after midnight if not sooner. Patchy ground fog in the typical rural spots will be possible late tonight depending on opacity of cirrus. Lows will fail to drop below 70 except in portions of the Shenandoah Valley and mountains.
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&& .SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/...
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The weak front will be arriving from the north on Saturday. For the most part it will only be a moisture boundary, with dew points of 5-8 deg lower to the north. At the same time, a shortwave trough will be crossing New England. The question for Saturday afternoon and evening will be if these features will provide enough lift to what could be a capped, hot atmosphere with poor mid-level lapse rates. Computer models aren`t overly zealous with QPF, but get the feeling we could at least see some scattered storms, especially east of the Blue Ridge. If so, they will be in an environment with sufficient CAPE and deep layer shear to become locally severe. These trends will need to be monitored. Heat will also be a concern for Saturday. Cirrus could again be a concern, but looks like a better chance for low elevations to reach the mid 90s, if not upper 90s. With elevated dew points ahead of the boundary, heat indices will exceed 100, especially from DC south. Am not sure we will reach Heat Advisory criteria of 105, so will pass these concerns along to subsequent shifts. Any convection will diminish by late Saturday evening, leaving a dry and mild overnight. The dew point boundary will never completely clear the area, likely leaving a gradient across the area for Sunday. To the north, despite continued heat again at least in the mid 90s, humidity values will be lower, and heat indices may not top 100. To the south, it will be a little muggier, and can`t rule out an isolated storm. However, the ridge will build east, likely capping most convection. Although models are generally dry for Sunday night, there will be theta-e advection aloft, so will have to watch the potential of showers/storms arriving from the west.
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&& .LONG TERM /MONDAY THROUGH FRIDAY/...
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Monday may very well rival Saturday as the most oppressive day over the next week as 850 mb temperatures remain in the 22-24 C range but dew points also climb back into the lower 70s F at the surface. High temperatures in the middle to upper 90s F (possibly near 100 F if full sunshine is realized) will result in heat index values in the 105-110 F range. In addition to the heat, an upper-level disturbance and its accompanying surface cold front will approach the area late in the day. Ample instability is expected given the high heat and humidity, so it seems quite plausible that the added energy with the approaching front could set off a round of strong thunderstorms Monday afternoon and evening. The high pressure area behind the front is not particularly strong which this time of year usually means the front will just weaken and dissipate over the region. This will result in only a slight degradation from Monday`s heat and humidity. It also means there will be a lingering chance of showers and thunderstorms each afternoon and evening through at least the middle of next week. No significant relief from the heat and humidity is forecast for at least the next week.
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&& .AVIATION /19Z FRIDAY THROUGH WEDNESDAY/...
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A few storms are developing to the west this afternoon, but coverage is somewhat iffy. Have used VCTS at MRB and IAD, although can`t totally rule out a storm at any terminal. Winds have been somewhat hard to peg with a weak trough over the area, but they should generally be from the SW or W. Some patchy ground fog may form again late tonight, but far from certain, so have just hinted with a 5 SM at CHO/MRB. Scattered storms are possible again Saturday afternoon and evening, with potential slightly higher (but far from certain) in the DC/Baltimore area. Thunderstorm chances lower, if nonexistent for Sunday. Light W/NW flow will become southerly late Sunday. Sub-VFR/gusty winds possible in any showers or thunderstorms that move across the area ahead of a cold front Monday afternoon and evening. Winds will be southwesterly around 10 knots outside of any convection through Monday evening, then should become more westerly Tuesday.
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&& .MARINE...
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SW flow 10-15 kt through this afternoon. Increase is still pegged this evening over the lower Maryland bay and adjacent Potomac. A Small Craft Advisory remains in effect for this area for this evening and overnight. Am not certain at this time if the stronger winds will be any more widespread than this. Lighter (and somewhat variable) winds are expected Saturday through Sunday, with southerly flow perhaps increasing Sunday night. The best chance for scattered thunderstorms will be Saturday afternoon and evening. A weakening cold front will approach the waters by late Monday. Southerly channeling ahead of the front may result in Small Craft Advisory level wind gusts on portions of the waters Monday afternoon and night. In addition, any thunderstorms that develop Monday afternoon and evening would likely produce gusty winds.
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&& .CLIMATE... Record highs and warm lows for the upcoming weekend... Today... DCA...103 in 1926...83 in 2011 BWI...106 in 2011...81 in 2011 IAD...105 in 2011...76 in 2011 Saturday... DCA...102 in 2011...84 in 2011 BWI...102 in 2011...79 in 1978 IAD... 99 in 2011...76 in 2011 Sunday... DCA...101 in 2010...84 in 2011 BWI...101 in 2010...82 in 2010 IAD... 99 in 2010...78 in 2010 Monday... DCA...100 in 1930...79 in 1965 BWI...100 in 2010...77 in 1887 IAD... 98 in 2010...76 in 1965 && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory from 6 PM this evening to 6 AM EDT Saturday for ANZ534-537-543. && $$ SYNOPSIS...ADS NEAR TERM...ADS SHORT TERM...ADS LONG TERM...DFH AVIATION...ADS/DFH MARINE...ADS/DFH CLIMATE...HTS

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