AFDLWX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC 330 PM EDT THU JUL 7 2011 .SYNOPSIS... A COLD FRONT OVER THE NORTHERN MID-ATLANTIC STATES WILL MOVE SOUTHWARD TOWARD NORTHERN MARYLAND TODAY BEFORE STALLING OVER THE AREA THROUGH FRIDAY. SURFACE HIGH PRESSURE WILL MOVE INTO THE REGION THIS WEEKEND. THE MID-ATLANTIC STATES WILL BECOME SITUATED BETWEEN HIGH PRESSURE OVER THE SOUTHEASTERN STATES AND LOW PRESSURE OVER EASTERN CANADA EARLY NEXT WEEK. && .NEAR TERM /THROUGH FRIDAY/... A W-E ORIENTED COLD FRONT CONTINUES TO SLOWLY PROGRESS SWD ACROSS SRN PA. DAYTIME DESTABILIZATION HAS RESULTED IN NUMEROUS SHOWERS AND STORMS DEVELOPING ALONG AND NORTH OF THE MASON-DIXON LINE SO FAR THIS AFTN....WHERE BOUNDARY-LAYER CONVERGENCE IS MAXIMIZED ALONG THE FRONT. THE MOST WIDESPREAD ACTIVITY WILL GRADUALLY SHIFT SWD LATE THIS AFTN AND EVE AS THE FRONT MOVES OVER NRN MD. HAVE INCREASED POPS ACCORDINGLY TO LIKELY/CATEGORICAL ALONG AND NORTH OF A LINE FROM BALTIMORE TO WINCHESTER. FARTHER SOUTH...LACK OF FRONTAL FORCING WILL LIMIT COVERAGE OF CONVECTION THIS AFTN. STILL EXPECT SCT ACTIVITY TO IMPACT THE I-66 CORRIDOR AND ONLY ISO COVERAGE FAR SOUTH ACROSS CENTRAL VA AND LOWER SRN MD DURING THE LATE AFTN AND EVE. SBCAPES AROUND 2500 J/KG AND MID-LVL DRY AIR WILL SUPPORT THE POTENTIAL FOR ISO STRONG TO SEVERE MICROBURSTS. WITH WEAK DEEP-LAYER SHEAR AND LACK OF UPPER FORCING...STORMS SHOULD BE PULSE SEVERE. THE SLOW STORM MOTION AND PRESENCE OF THE SLOW-MOVING FRONT WARRANTS CONCERN FOR LOCALLY HVY RAINFALL AND ISO FLASH FLOODING NEAR THE BOUNDARY. INTENSITY OF SHOWERS AND STORMS SHOULD WANE AFTER SUNSET WITH THE LOSS OF HEATING. HOWEVER...ISO TO SCT CONVECTION MAY LAST INTO THE LATE EVE OR OVNGT...ESPECIALLY OVER THE NRN HALF OF THE CWA...AS THE FRONT STALLS OVER THE AREA. LOW TEMPS WILL RANGE FROM THE MID 60S ACROSS THE HIGH ELEVATIONS TO MID 70S IN THE CITIES AND ALONG THE COAST. CLOUD COVER SHOULD PREVENT WIDESPREAD AND DENSE FOG ERY FRI MRNG. AN UPPER TROUGH WILL APPROACH FROM THE OH VLY WHILE A STRENGTHENING UPPER-LEVEL JET STREAK MOVES INTO THE NORTHEAST STATES ON FRI. SLY FLOW AHEAD OF THE TROUGH WILL DRAW TROPICAL MOISTURE NWD INTO THE MID-ATLANTIC STATES WHILE THE SFC BOUNDARY REMAINS STALLED OVER THE AREA. SHOWER AND STORMS SHOULD INCREASE IN COVERAGE DURING THE MIDDAY AND AFTN HOURS AS SYNOPTIC LIFT INCREASES. THE MAIN CONCERN STARTING FRI AFTN WILL BE HEAVY RAINFALL AND FLASH FLOODING...ESPECIALLY WITH PWATS APPROACHING 2 STANDARD DEVIATIONS ABOVE NORMAL. SEE HYDRO DISCUSSION FOR MORE DETAILS. ANOTHER CONCERN WILL BE THE POTENTIAL FOR SEVERE TSTMS AS DEEP-LAYER SHEAR PROFILES INCREASE IN CONCERT WITH DAYTIME DESTABILIZATION. HOWEVER...THERE IS A HIGH DEGREE OF UNCERTAINTY FOR THE SEVERE TSTM POTENTIAL AS THE THREAT WILL DEPEND ON THE AMOUNT OF DAYTIME HEATING. CURRENT THINKING IS THAT CLOUD COVER ALONG AND NORTH OF THE FRONT WILL LIKELY LIMIT HEATING. CENTRAL VA AND SRN MD WILL MOST LIKELY SEE BREAKS IN THE CLOUDS AND THEREFORE BETTER INSTABILITY. && .SHORT TERM /FRIDAY NIGHT THROUGH SUNDAY NIGHT/... S/WV PIVOTS ACRS CWFA FRI EVE...TAKING AXIS OF HVY RAIN W/ IT. PWAT IN EXCESS OF 2 INCHES /CLS TO 2SD ABV NRML/ MOST LKLY CHO-DCA-NAK. FF WATCH WL CONT THRU 06Z...TO ACCT FOR POTENTIAL OF REPEATED ECHOES. FNT WL BE SOMEWHAT CAUGHT IN MID LVL FLOW...SO DONT XPCT IT TO MV THRU QUICKLY. HENCE...LKLY POPS WL STRETCH INTO THE OVNGT HRS. BUT...STABILITY WL INCR THRU THE NGT...AND WL SHIFT EMPHASIS AWAY FM THUNDER FOR THE OVNGT HRS. SHRA CUD LINGER INTO SAT MRNG...BUT THEN HIPRES WL BLD OVR THE GRTLKS...DRIVING CDFNT FAR ENUF S TO CARVE OUT A DRY WKND. H8 TEMPS DONT DROP MUCH...SO MAXT FCST WL BE SIMLR TO PAST FEW DAYS /NEAR 90F/...BUT W/ DEWPTS IN THE 60S. MIN-T CLOSE TO MOS MEAN. && .LONG TERM /MONDAY THROUGH THURSDAY/... XTNDD SYNOPTIC PTTN FEATURES BROAD MID CONUS H5 RDG AND DOWNSTREAM TROF SOMEWHERE ALONG THE ERN SEABOARD OR IN THE WRN ATLC. THAT WL LEAVE A SFC FNT STRETCHED FM W TO E SOMEWHERE IN THE MID ATLC OR CAROLINAS...WHILE TRANSITORY SFC HIPRES PROTRUDES FM THE GRTLKS/NEW ENGLND. THE INTERACTION BTWN THESE TWO FEATURES WL DICTATE DIRECT IMPACT ON SENSIBLE WX W/IN CWFA. AS LONG AS THE FNT IS NEARBY...NEED TO CONSIDER CHC TSRA DAILY...W/ AN EMPHASIS PLACED ON LT AFTN-ELY EVNG. GRIDS WL STRIKE THAT BALANCE...W/ MAXT ABV CLIMO MON-TUE WHILE PRIMARY INFLUENCE COMES FM UPA RDG...THEN TWD CLIMO WED-THU AS SFC HIPRES POTENTIALLY DRIVES FNT SWD. && .AVIATION /20Z THURSDAY THROUGH TUESDAY/... SHRA/TSRA ACTIVITY NEAR THE MASON-DIXON LINE SHOULD PROGRESS SWD TOWARD MRB/BWI/MTN AS A COLD FRONT SLOWLY PUSHES INTO NRN MD DURING THE LATE AFTN/EVE. COVERAGE/CHANCES OF CONVECTION DECREASE FARTHER SOUTH. FOG SHOULD BE LESS WIDESPREAD AND DENSE THAN LAST NGT WITH CLOUD COVER HIGHER TNGT. HOWEVER...PATCHY FOG STILL EXPECTED. THE BIGGEST CONCERN FOR IFR FOG WILL BE ACROSS CHO DUE TO MORE BREAKS IN CLOUD COVER. SHOWERS AND STORMS WILL INCREASE AGAIN DURING THE LATE MRNG AND AFTN FRI AS THE FRONT STALLS OVER THE AREA. ACTIVITY SHOULD BE MORE WIDESPREAD THAN TDA WITH BETTER FORCING FROM AN APPROACHING UPPER TROUGH. PERIODS OF FLIGHT RESTRICTIONS FROM HEAVY SHRA/TSRA WILL BE LIKELY ACROSS ALL TERMINALS FRI AFTN. POOR FLYING CONDS FRI NGT IN NMRS SHRA/TSRA. PREVAILING MVFR W/ LCL IFR XPCTD. A WK CDFNT WL PUSH SEWD OVNGT..WHICH WUD END SHRA...BUT LINGERING LLVL MSTR MAY RSLT IN FOG DVLPMNT. THAT CUD LAST INTO SAT MRNG. THEREAFTER...VFR THRU THE WKND...AND INTO ELY NXT WK. && .MARINE... WINDS WILL REMAIN BELOW SCA LVLS TNGT AND FRI. SCT TSTMS WILL IMPACT THE WATERS THRU THIS EVE...ESPECIALLY IN THE FAR NRN BAY. THE STRONGEST STORMS COULD PRODUCE LOCALLY GUSTY WINDS. WIDESPREAD SHOWERS AND STORMS ARE EXPECTED AGAIN ON FRI. NMRS TSRA FRI NGT MAY BE RESPONSIBLE FOR LCLLY HIER WNDS/WVS. OTRW...NW WNDS XPCT TO REMAIN AOB 15 KT THRU THE WKND. STRONGEST WNDS WL COME SAT AS CDFNT PUSHES S. RTN FLOW WL BEGIN LT SUN. && .HYDROLOGY... 12Z GUIDANCE COMING INTO BETTER AGREEMENT OF A HEAVY RAINFALL EVENT FRI AFTN AND EVE AS A TROUGH OF LOW PRESSURE WILL ADVANCE TOWARD THE REGION. ALTHOUGH ANTECEDENT CONDITIONS ARE RELATIVELY DRY/FFG HIGH...DECENT SYNOPTIC LIFT...A MOIST AIRMASS EMANATING FROM THE TROPICS...SLOW STORM MOTION AND THE PRESENCE OF A STATIONARY BOUNDARY ARE AMONG MANY FACTORS CONTRIBUTING TO A POTENTIAL FLASH FLOOD EVENT. HAVE ISSUED A FLASH FLOOD WATCH FOR MOST OF THE AREA EXCEPT WRN MD...THE ERN WEST VIRGINIA PANHANDLE AND EXTREME NW VIRGINIA. HEAVY RAINFALL WILL NOT BE WIDESPREAD...AND LIKELY BE FOCUSED ALONG THE FRONT. THEREFORE...THE EXACT LOCATION OF THE FRONT WILL BE KEY TO DETERMINING WHO GETS MESOSCALE BANDING OF HEAVY RAINFALL. SEE FLASH FLOOD WATCH FOR MORE DETAILS ON EXPECTED PRECIP AMOUNTS. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...FLASH FLOOD WATCH FROM FRIDAY AFTERNOON THROUGH LATE FRIDAY NIGHT FOR DCZ001. MD...FLASH FLOOD WATCH FROM FRIDAY AFTERNOON THROUGH LATE FRIDAY NIGHT FOR MDZ004>007-009>011-013-014-016>018. VA...FLASH FLOOD WATCH FROM FRIDAY AFTERNOON THROUGH LATE FRIDAY NIGHT FOR VAZ025>027-029>031-036>040-042-050>057-501>504. WV...FLASH FLOOD WATCH FROM FRIDAY AFTERNOON THROUGH LATE FRIDAY NIGHT FOR WVZ505-506. MARINE...NONE. && $$ SYNOPSIS...HTS