AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE RALEIGH NC 1107 - TopicsExpress



          

AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE RALEIGH NC 1107 AM EDT MON JUN 17 2013 .SYNOPSIS... A COLD FRONT WILL APPROACH FROM THE NORTHWEST TODAY AND ACROSS OUR REGION LATE TUESDAY. && .NEAR TERM /THROUGH MONDAY NIGHT/... AS OF 11 AM MONDAY... TODAY: A WEAK SHEAR AXIS IS CURRENTLY MOVING ACROSS THE AREA THIS MORNING IN THE WESTERLY MID LEVEL FLOW... HELPING TO SPREAD MOSTLY CLOUDY TO OVERCAST SKIES ACROSS MOST OF THE AREA... WITH EVEN A FEW SHOWERS AND STORMS ACROSS THE PIEDMONT. THIS SHEAR AXIS IS EXPECTED TO SLOWLY SHIFT EASTWARD THIS MORNING AND EARLY AFTERNOON... WITH PERHAPS SOME SHOWERS AND STORMS DEVELOPING ALONG AND TO THE EAST OF THE FEATURE. IN FACT... SOME GUIDANCE SUGGEST THAT WE COULD SEE SOME UPWARD TREND IN INTENSITY... WITH THE SHOWERS AND STORMS ASSOCIATED WITH THIS DISTURBANCE... BRIEFLY ACROSS OUR FAR EAST/SOUTHEAST... BEFORE THE WAVE PUSHES TO THE EAST OF OUR AREA. HOWEVER... THE MAIN FOCUS FOR SHOWERS AND STORMS TODAY IS EXPECTED TO BE ASSOCIATED WITH ANOTHER WEAK MID LEVEL DISTURBANCE EXPECTED TO MOVE ACROSS THE AREA THIS AFTERNOON INTO THE EVENING... CURRENTLY SUPPORTING SHOWERS AND STORMS ACROSS THE EASTERN TN... NORTHERN GA AND FAR WESTERN NC. HI-RES MODELS SHOW THIS ACTIVITY MOVING INTO THE WESTERN PORTION OF THE AREA SOMETIME IN THE 18-21Z TIME FRAME... THEN SPREADING EASTWARD THROUGH THE REST OF THE AFTERNOON AND EVENING... WITH ANY REMAINING ACTIVITY LATE THIS EVENING DWINDLING IN COVERAGE AND INTENSITY WITH NOCTURNAL COOLING. WRT A SEVERE THREAT... WITH SKIES MOSTLY CLOUDY AND PW`S IN THE 1.7 TO 2.0 INCH RANGE TODAY... THINK WE WILL ONLY SEE MODEST MLCAPE VALUES AT BEST... GENERALLY AROUND 1000 J/KG DURING PEAK HEATING. DEEP LAYER SHEAR IS EXPECTED TO REMAIN WEAK... IN A GENERALLY WEAK WESTERLY MID LEVEL FLOW. THUS... EXPECT THE SEVERE POTENTIAL TO BE LOW... WITH THE HIGHEST THREAT OF ANY HAZARDOUS WEATHER FROM POSSIBLE HEAVY SHOWERS AND STORMS MOVING OVER AN URBAN AREA (FLASH FLOODING). HOWEVER... THINK COVERAGE WILL REMAIN MORE SCATTERED TODAY AND GIVEN THE MEAN FLOW OF 10 TO 20 KTS... THINK THE FLASH FLOODING THREAT WILL BE LOW TOO. ALSO... IT IS WORTH NOTING THAT SOME OF THE HI-RES MODELS DO APPEAR TO SHOW SOME TYPE OF ORGANIZATION LATE IN THE DAY WITH THE SHOWERS AND STORMS (THINK THIS IS A BIT OVERDONE THOUGH). TEMPS TODAY WILL BE HAMPERED BY THE EXTENSIVE CLOUD COVER. THUS... THINK PREVIOUS FORECAST HIGHS OF 83 TO 87 STILL LOOK GOOD. TONIGHT: BOTH THE GFS AND ECM SHOW A LULL ACROSS THE NORTHERN TIER TONIGHT ACROSS THE NORTHERN TIER IN SUBSIDENCE BEHIND THE INITIAL UPPER SHORT WAVE. THUNDER WILL BE TOUGH TO PRODUCE GIVEN SUBSIDENCE AND UNFAVORABLE NOCTURNAL TREND. WILL TREND POPS FOR SHOWERS TOWARDS THE SOUTHEAST. MINS WILL BE PERSISTENCE AND ON THE MILD SIDE...UPPER 60S. && .SHORT TERM /TUESDAY THROUGH WEDNESDAY NIGHT/... AS OF 240 AM MONDAY... A SERIES OF DISTURBANCES ALOFT...WHICH HAVE TRENDED WEAKER IN NWP GUIDANCE DURING THE PAST FEW DAYS AND WHOSE TIMING IS UNCERTAIN GIVEN THE PROBABILITY OF BEING MODULATED BY UPSTREAM CONVECTION...WILL MIGRATE THROUGH THE BASE OF A BROAD TROUGH ALOFT EAST OF THE ROCKIES...AND ACROSS THE CENTRAL APPALACHIANS/NC THROUGH WED. MEANWHILE AT THE SURFACE...A PRE-FRONTAL TROUGH OVER THE MIDDLE ATLANTIC REGION THIS MORNING WILL STRENGTHEN THROUGH TUE...THEN BE OVERTAKEN BY A COLD FRONT THAT WILL SETTLE SLOWLY SOUTHWARD FROM THE MASON DIXON LINE TUE EVENING TO THE NC/SC LINE BY WED EVENING. PRECIPITABLE WATER VALUES WILL CONSEQUENTLY REMAIN ABOVE AVERAGE...BETWEEN 1.5 AND 1.8 INCHES... UNTIL FRONTAL PASSAGE. THE DEEP MOISTURE AND ASSOCIATED UNINHIBITED CONDITIONAL INSTABILITY... WHICH IS FORECAST TO MAXIMIZE IN THE 1000-1500 J/KG EACH AFTERNOON...WILL SUPPORT NUMEROUS TO WIDESPREAD THUNDERSTORMS...WITH COVERAGE DIMINISHING WITH NOCTURNAL COOLING TUE NIGHT...AND THEN FROM NORTH TO SOUTH WITH THE FRONTAL PASSAGE ON WED. GIVEN THE EXPECTATION OF WEAKER DISTURBANCES THAN PREVIOUSLY INDICATED...NOTED ABOVE...MID LEVEL FLOW AND ASSOCIATED BULK SHEAR VALUES ARE NOW FORECAST TO PEAK ONLY IN THE 20-25 KT RANGE...HIGHEST WED. AS SUCH...ALREADY MINIMAL SEVERE POTENTIAL WILL BE FURTHER MUTED UNLESS POCKETS OF STRONGER INSOLATION AND MODERATE INSTABILITY ARE REALIZED...IN WHICH CASE A FEW STRONG-MARGINALLY SEVERE WET MICROBURSTS WOULD BE PROBABLE. A MORE LIKELY SCENARIO REMAINS A THREAT OF DOWNPOURS AND LOCALIZED FLOODING IN TRAINING CONVECTION...OWING TO WSW MEAN FLOW ALIGNED PARALLEL TO THE MYRIAD OF UNDERLYING SURFACE BOUNDARIES (TROUGH/EFFECTIVE FRONT-OUTFLOW /ACTUAL FRONT...AND TO BACKBUILDING INDICATED BY SMALL FORECAST CORFIDI VECTORS. ANY SUCH FLOODING WILL NOT BE WIDESPREAD...AND INSTEAD CONFINED MOST LIKELY TO URBAN AREAS SINCE THE ANDREA-SOAKED SOIL HAS DRIED IN THE PAST WEEK TO 10 DAYS...AND RESULTANT FLASH FLOOD GUIDANCE VALUES HAVE INCREASED...INTO THE 2.5 TO 3 INCH RANGE FOR ONE HOUR FOR EXAMPLE. PROVIDED THERE ARE AT LEAST A FEW BREAKS IN OTHERWISE MOSTLY CLOUDY SKIES AND CONVECTION...TEMPERATURES SHOULD CLIMB INTO THE LOWER TO MIDDLE 80S...WARMEST SANDHILLS TUE. OTHERWISE...UPPER 70S WOULD BE PROBABLE (IE. IN AREAS THAT EXPERIENCE MORE FREQUENT SHOWERS AND STORMS). LOWS TUE NIGHT MOSTLY IN THE UPPER 60S...TO AROUND 70 DEGREES SOUTH...COOLING A CATEGORY TO TWO IN COOLER POST-FRONTAL NE FLOW WED NIGHT. && .LONG TERM /THURSDAY THROUGH SUNDAY/... AS OF 305 AM SUNDAY... THU THROUGH FRI: BELOW AVERAGE TEMPERATURES - GENERALLY IN THE LOWER TO MIDDLE 80S - IN WEAK POST-FRONTAL HIGH PRESSURE AND ASSOCIATED NE TO EAST LOW LEVEL FLOW AND STABILITY...WILL THEN PREVAIL THROUGH THE END OF THE WEEK. WILL ACCORDINGLY MAINTAIN THE FLAVOR OF PREVIOUS GENERALLY DRY FORECASTS FOR THU-FRI...EXCEPT FOR THE SLIGHTEST OF CHANCES ALONG THE FAR SOUTHERN AND WESTERN TIER...WHERE BOUNDARY LAYER MOISTURE WOULD SUPPORT A LATE DAY SHOWER OR STORM. EVEN THAT SLIGHT CHANCE MAY BE OVERDONE GIVEN AN APPARENT LACK OF TRIGGER AMIDST DIVERGENT ANTICYCLONIC FLOW WITHIN THE AFOREMENTIONED SURFACE RIDGE AXIS. THIS WEEKEND: A SHALLOW UPPER LOW WILL HAVE DRIFTED INTO THE SOUTHEAST STATES BY THE START OF THE WEEKEND...BUT INSTEAD OF LIFTING TOWARD THE EAST COAST...GUIDANCE SUGGESTS THIS FEATURE WILL MOVE WESTWARD AROUND THE SOUTHERN PERIPHERY OF A DEVELOPING MID-UPPER RIDGE FROM THE SOUTHERN PLAINS TO THE MIDDLE ATLANTIC REGION. AS SUCH...TEMPERATURES...AND HUMIDITY LEVELS WILL GRADUALLY INCREASE IN INCREASINGLY SE TO SOUTHERLY RETURN FLOW. THE INCREASING WARMTH AND HUMIDITY WILL ALSO SUPPORT A SLIGHT UPTICK IN PROBABILITY OF PRECIPITATION...THOUGH STILL BELOW CLIMATOLOGICAL VALUES IN THE 20TH PERCENTILE GIVEN THE AFOREMENTIONED EXPANSION OF RIDGING ALOFT...AND LACK OF LOW LEVEL FOCI OTHER THAN PERHAPS THE SEA BREEZE. && .AVIATION /06Z MONDAY THROUGH FRIDAY/... AS OF 630 AM MONDAY... CONDITIONS WILL BE VFR OUTSIDE OF CONVECTION THROUGH LATE TONIGHT. CONVECTION MOVING EAST OFF THE MOUNTAINS INTO THE NORTHERN PIEDMONT THIS MORNING WILL WEAKEN AS IT MOVES TOWARDS THE TRIANGLE AND INTO MORE STABLE AIR. HEATING WILL NOT BE STRONG DUE TO MID LEVEL CEILINGS...BUT WEAKLY FORCED FLOW WILL PRODUCE SCATTERED SHOWERS AND WIDELY SCATTERED THUNDER THIS AFTERNOON....CONFIDENCE IN THE TIMING IS LOW AND THE COVERAGE NOT EXPECTED TO BE SUBSTANTIAL ENOUGH TO WARRANT INCLUSION IN THE TAFS. EXPECT A LULL OVERNIGHT ACROSS THE NORTHERN TIER IN SUBSIDENCE BEHIND A WEAK UPPER SHORTWAVE. VERY MOIST AIRMASS WILL BE FAVORABLE FOR FOG AND PERHAPS SOME IFR/LIFR CEILINGS IF WE GET SOME BREAKS TO ALLOW RADIATIONAL COOLING AND WINDS CALM IN THE PREDAWN. ADDITIONAL DISTURBANCES WILL BE CROSSING THE AREA ALONG WITH A WEAK COLD FRONT TUESDAY INTO WEDNESDAY...WITH PERIODS OF SHOWERS AND STORMS PRODUCING SUB-VFR CONDITIONS. VFR CONDITIONS ARE GENERALLY EXPECTED FROM THURSDAY ONWARD... OUTSIDE OF POSSIBLY SOME EARLY MORNING STRATUS AND OR FOG DURING THE PRE-DAWN HOURS OF EACH MORNING. && .RAH WATCHES/WARNINGS/ADVISORIES... NONE. &&
Posted on: Mon, 17 Jun 2013 16:04:28 +0000

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