Scientific Forecaster Discussion

000 fxus62 kgsp 231447 afdgsp Area forecast discussion National Weather Service Greenville-Spartanburg SC 1047 am EDT Tue Apr 23 2019 Synopsis... warm and dry high pressure will be in control of our weather through Wednesday. A cold front will bring rain and possibly thunderstorms to the region sometime late Thursday into Friday. Active weather may return Sunday with another frontal passage. && Near term /through tonight/... as of 1045 am EDT tuesday: forecast generally on track this am. Cirrus will continue to increase as moisture spills in ahead of an approaching short wave and moves over the upper ridge across the area. Surface high pressure moves east today with a relatively strong Lee trough developing. This will help produce increasing SW winds near the trough with low end gusts possible. Lighter SW wind expected in the Piedmont and along the I-77 corridor. This wind flow also produces a rare westerly wind in the French Broad valley, with low end gusts possible there as well. The bigger story with these winds will be the downsloping conditions and rising temps, which will approach values seen outside of the mountains. Although instability develops during the afternoon, low levels remain relatively dry. This combined with the warm temps keeps a cap in place with very high level of free convection levels. No convection is expected even with the convergence along the Lee trough. Highs will be around 10 degrees above normal, around 15 above normal in the French Broad valley. A short wave moves by to our north tonight dampening the ridge but with only small height falls. A cold front approaches from the north but never quite gets here. Despite some forcing and developing northwest flow, low levels remain quite dry so no precip expected, though there will be an increase in mid and high level clouds. This should keep fog formation and radiational cooling to a minimum, with lows expected to be around 10 degrees above normal. && Short term /Wednesday through Thursday night/... as of 315 am EDT tuesday: fairly quiet weather Wednesday into Wednesday night under flat ridging with a couple of disturbances approaching from the west. Model guidance is now is better agreement that a weakening front will stall just to the north during this period with the main impact being some increase in low to mid clouds to northern areas, although can't completely rule out an isolated shower or two across the northern mountains. Downsloping flow to the east of the mountains will provide warm and dry conditions with humidities possibly briefly dipping below 30 percent in the afternoon despite some high clouds passing overhead. Highs Wednesday will be well in to the 80s for all but mtn valleys, but even here highs will approach 80. Despite the insulating effects of high clouds continuing if not becoming thicker, low dewpoints will likely allow temps to drop off into the 50s to near 60 Wednesday night. Model guidance is now in agreement that northern and southern stream energy will merge while pushing a cold front through the region Friday. Increasing moist southwesterly flow ahead of this will bring increasing high clouds into Thursday, which will lower and thicken through the day, especially across the mountains. The increasing moisture and diurnal heating should be enough to spawn isolated to widely scattered showers with thunder possible across northern areas where there is less capping at low levels and a bit better forcing. Shower and thunderstorm activity may wane or becoming more focused on upslope areas such as the Southwest Mountains and Escarpment. Increasing cloud cover with knock a few degrees off highs with 70s mtn valleys to low to maybe mid 80s east of the mountains. Rainfall amounts through Thursday night should be light and sporadic outside of thunderstorms. && Long term /Friday through Monday/... as of 340 am EDT tuesday: about average confidence exists for large scale features including a cold front to push through the region sometime Friday. However details remain fuzzy at best. As mentioned in the short term, model guidance has come together in merging northern and southern stream disturbances. The resultant timing and strength of the cold front is still up for grabs. The ec (which first picked up on the merging streams) has trended stronger with the feature with a slight negative tilt to the trough and now shows sufficient instability, shear, and upper support for at least some level of severe thunderstorm threat. Meanwhile the GFS continues to be more progressive and weaker with virtually no severe threat. Current thinking and forecast leans towards the ec kind of solution, with better shower and thunderstorm chances Friday, lingering at least into the afternoon for much of the area. Given the severe threat is still marginal if one buys fully into the new ec solution, will await better run to run to model consistency before taking further steps (such as including in hwo). Hydro concerns continue to be limited, although will bear some watching, especially given the latest run of the ec. Drying trends behind the front will likely support mostly sunny skies into Saturday. Active weather may continue Sunday into early next week as a couple more frontal systems may push into the area. Current model guidance is not impressed with these features and therefore current forecast keeps any rain chances low. Cloudy skies with rain Friday follow by clearing skies offset by cold air advection behind the cold front Saturday will support highs in the 70s, possibly warming into the 80s Sunday into early next week under rising 500 mb heights. && Aviation /15z Tuesday through Saturday/... at kclt and elsewhere: VFR through the period at the taf sites. Despite some variability, winds will generally remain out of the S to SW outside of the mountains. Speeds pick up in the afternoon with low end gusts possible, especially closer to the developing Lee trough. Still expecting a rare westerly wind at kavl during the day with low end gusts possible. Direction returns to a normal north-northwest during the evening but with lowering speeds. Cirrus increases and thickens through the period with mid level clouds possible overnight. Outlook: expect VFR conditions Wednesday. Periods of showers and thunderstorms associated with a cold front will bring the potential for restrictions Thursday and Friday. VFR returns for the weekend as high pressure builds in behind the cold front. Confidence table... 14-20z 20-02z 02-08z 08-12z kclt high 100% high 100% high 100% high 100% kgsp high 100% high 100% high 100% high 100% kavl high 100% high 100% high 100% high 100% 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 && Gsp watches/warnings/advisories... Georgia...none. NC...none. SC...none. && $$ Synopsis...Munroe

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