AFDLWX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC 923 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 TOMORROW. 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/... NEARLY STATIONARY BNDRY HAS BEEN SKEWED TO A MORE NW-SE ORIENTATION DUE TO CNVCTN ACRS NE MD...NOW DRAPED ROUGHLY ALONG POTOMAC RIVER. SVRL WET MICROBURST ACRS NRN CWA EARLIER THIS EVE...BUT WITH DIURNAL INSTABILITY WANING...STORM INTENSITY HAS BEEN AND SHUD CONT DCRSG. HVY RN...2-4 INCHES ACRS PORTIONS OF INNER BALTIMORE METRO HAS CAUSED LOCALIZED FLASH FLOOD PROBLEMS...WHICH TOO SHUD DIMINISH AS CNVCTN WEAKENS. POPS GRADUALLY LOWER TO CHC/SLIGHT CHC AFTR MIDNIGHT...WITH BEST PROBS ACRS NRN ZONES...CLOSER TO SFC BNDRY. PREV DISCUSSION FOLLOWS... 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 /01Z FRIDAY THROUGH TUESDAY/... THUNDERSTORMS CURRENTLY IMPACTING THE BWI AND MTN TERMINALS WILL DIMINISH THROUGH MIDNIGHT GIVING WAY TO MOSTLY CLOUDY SKIES. A LOW PRESSURE SYSTEM MOVING NORTHWARD ALONG THE ATLANTIC COAST COMBINED WITH THE SLOW MOVING FRONT CURRENTLY ACROSS THE AREA WILL PRODUCE WIDESPREAD SHOWERS AND THUNDERSTORMS THROUGH THE DAY TOMORROW. EXPECT IFR CONDITIONS PERIODICALLY AT ALL AIRPORTS DUE TO HEAVY RAIN OBSTRUCTING VISIBILITIES AND LOW CIGS AT 2000 FT OR LOWER. OVERNIGHT...FOG FORMATION WILL REDUCE VISIBILITIES ACROSS ALL TERMINALS. OVERALL...LIGHT WIND SHIFTING FROM SOUTHWEST TO NORTHERLY WILL PREVAIL ACROSS THE REGION. HOWEVER...STRONG WIND GUSTS IN SHOWERS AND THUNDERSTORMS COULD IMPACT TERMINALS WELL INTO TOMORROW EVENING. PREV DISCUSSION FOLLOWS... 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... TSTMS MAINLY CONFINED TO NRN PORTIONS OF BAY TNGT...OTHERWISE LGT WINDS XPCD. PREV DISCUSSION FOLLOWS... 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/SBK