Scientific Forecaster Discussion

Area forecast discussion National Weather Service Greenville-Spartanburg SC 243 PM EDT Tuesday Jul 26 2016 Synopsis... broad surface high pressure will linger over the southeast, before sliding offshore as a surface low develops and moves northeast across the Midwest and Great Lakes regions. A surface front will remain off to our north and linger within a region between the Ohio Valley, lower Great Lakes, and mid-Atlantic regions. This front may finally move into our area early next week. && Near term /through Wednesday/... as of 200 PM...the large 500 mb high moves little thru Wednesday, but weakens slightly, as a series of shortwaves try to flatten the northern side of the ridge. Meanwhile, a quasi-stationary front remains draped along the Ohio Valley to the northern mid-Atlantic. The convective trends are similar to yesterday, perhaps a tad slower, and mainly confined to the high terrain so far. Guidance still in agreement on scattered showers and storms propagating/developing east into the Piedmont late afternoon into early evening. Pops were changed little from previous forecast in showing these trends. Severe threat still looks marginal, but SBCAPE on the mesoanalysis Page is 2500-3000 j/kg with dcape 800-1200 j/kg. So a few pulse severe storms will be possible. Convection should wane this evening with loss of heating, leaving a fair amount of debris cloudiness within the weak flow aloft. Min temps will be generally 5-7 degrees above normal. The other issue is the heat, with the latest obs coming in with heat indices a little on the high side. A couple of 105 degrees out there in the Piedmont at ASOS sites. Wednesday looks similar today. Perhaps a degree or two warmer, but with a little more mixing out of the dewpoints. The latest heat index forecast is in the 96-104 range again across the Piedmont during the hottest part of the day. A heat advisory will be considered for Wednesday, but will allow the midnight shift to make the final call. Otherwise on Wednesday, the moisture plume will actually lift back north slightly, as result in convection being more confined to the mountains and I-40 corridor. The Georgia/SC Piedmont may not see much of any deep convection, as weaker mid-level lapse rates pivot around the the ridge from the south. So will feature a tighter pop gradient from less than slight chc pop in the lower Piedmont to likely along the NC mountain peaks. && Short term /Wednesday night through Friday/... as of 230 PM EDT tuesday: not much change to the overall synoptic pattern from what we've been advertising the past few days. The subtropical ridge will continue to dominate, with moisture associated with a tropical weakness moving onshore today moving up the lower Mississippi Valley at the start of the period. Meanwhile a broad shortwave will push across the northern tier of the country, pulling the moisture associated with the lower Mississippi weakness to the north toward the Ohio Valley. Surface high pressure will continue to pump a warm and moist air into the region with in general a fairly typical summertime pattern continuing, but with the additional moisture creeping into northwest zones from the approaching wave/front. Additionally, a Lee trough progged to strengthen tomorrow will continue into the short term, which will provide additional focus for convective development. Deep layer shear will marginally increase with the approach of the shortwave, and combined with healthy SBCAPE values between 2000-3000 j/kg, the continued marginal threat outlined on the swody3 for northern zones looks good. As has been the case, low-level lapse rates will continue to support isolated wet microburst threat, but mid-level lapse rates not high enough for much in the way of hail concerns. As we push into Friday, vort Max should ride up the southwest flow aloft ahead of the wave, with the GFS farther south with an area of enhanced qpf, more over the northern mountains, but the European model (ecmwf) farther north up in WV. Either way this doesn't help the upstate and other areas in continued drought, but with the additional synoptic lift, could certainly see more widespread coverage across NC. Have not reflected the GFS solution in pops for Friday and will have to reevaluate as operational guidance comes into better consensus. Temperatures will remain a category or so above seasonal normals, but could be adjusted further one way or another depending on model evolution of precipitation/cloud shields. Humidity will remain high with areas in extreme southeast zones continuing to flirt with heat advisory levels, though should see some minimal improvement on Friday. && Long term /Friday night through Tuesday/... as of 200 PM Tuesday...the broad scale pattern will not change much over the ext range. An h5 trof axis remains west of the fcst area and will likely get pulled slowly NE late in the period. This will advance a weak sfc front toward the County warning forecast area...perhaps pushing into/across the mtns by Mon. The flow remains weak and generally W/ly within the h9/h7 mech lift along with enhanced llvl moisture from the nearby front will support above normal pops across the higher terrain. Downsloping in the Lee will help suppress convec activity...thus near normal pops will be maintained over the non/mtns. Warm temps continue in a relatively stagnant pattern with Max temps will reaching around or a couple degrees above normal each day. Mins will also be held a Cat or so above climo. && Aviation /19z Tuesday through Sunday/... at kclt and elsewhere: convection getting a slightly slower start than yesterday, and is mainly confined to the ridgetops so far. However, guidance still in good agreement on there being enough flow to steer some of the activity to the east into the Piedmont, especially along and north of a line from gmu-gsp-uza. So I will continue to keep a tempo at kclt starting at 21z, which is a little earlier than previous thinking. The other sites will feature thunderstorms in the vicinity during the afternoon and early evening. Winds should favor a SW direction, except northwest at kavl, and variable around thunderstorms and rain. Later tonight, there should be a fair amount of debris cloudiness lingering well into the night. And not expecting much fog/stratus. However, with expected better coverage of rain in the mountains than last couple days, kavl may have a better shot at some restrictions late tonight. Similar conditions are expected across the area for Wednesday. Outlook: a plume of deep moisture will gradually build across the western Carolinas and NE Georgia through the late week. Scattered rain showers and thunderstorms and rain will be possible each afternoon and evening, coverage the greatest across the mtns. Pre dawn fog and low clouds will be possible over areas of recent rainfall. Confidence table... 18-24z 00-06z 06-12z 12-18z kclt high 100% high 100% high 100% high 100% kgsp high 100% high 100% high 100% high 100% kavl high 100% high 100% high 94% high 97% khky high 100% high 100% high 100% high 100% kgmu high 100% high 100% high 100% high 100% kand high 100% high 100% high 100% high 100% The percentage reflects the number of guidance members agreeing with the scheduled taf issuance flight rule category. Complete hourly experimental aviation forecast consistency tables and ensemble forecasts are available at the following link: Www.Weather.Gov/gsp/aviation && Climate... records for 07-26 Maximum temperature min temperature station high low high low ------- ---------- ---------- ---------- ---------- kavl 94 1949 72 1911 71 2012 49 1911 1940 2010 kclt 100 2005 74 1920 76 1992 60 1904 1940 1940 1914 1936 kgsp 99 2010 76 1920 76 2005 53 1911 1995 1940 1987 Records for 07-27 Maximum temperature min temperature station high low high low ------- ---------- ---------- ---------- ---------- kavl 97 1952 70 1946 72 1936 48 1911 1925 kclt 103 1940 74 1926 76 1944 57 1920 1940 1936 kgsp 103 1940 70 1946 75 2012 54 1911 1944 && Gsp watches/warnings/advisories... Georgia...none. NC...none. SC...none. && $$ Synopsis...sbk near term...Arkansas short term...tdp long term...sbk aviation...Arkansas climate...

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