Area forecast discussion National Weather Service Pittsburgh PA 547 PM EDT Wednesday may 22 2013 Synopsis... shower and thunderstorm chances will continue until a cold front passes through Thursday evening. High pressure returns for the weekend. && Near term /through Thursday/... mesoscale data shows ml convective available potential energy 1500 j/kg over WV into western PA. Mesoscale/near term model data shows shear should increases very slightly into the evening...maintaining some risk for a few severe thunderstorms. Atmosphere should begin to stabilize in the forecast area after sunset. Another surge of moisture seen on satellite is experimental to move northward keeping shower chances into the overnight period. Still a chance of a thunderstorm with some elevated instability but severe threat should end. A cold front will aprch from the west Thursday bringing addnl shwrs/tstms. Model forecasts show instability should be minimal and this seems reasonable given expd cloud cover. && Short term /Thursday night through Saturday/... a rapid transition to much colder conditions takes place during this period as the consensus of the more reliable global model systems suggesting the middle-level system tracking through the region should close off as it moves over Pittsburgh and slowly oozes southeastward. The closed solutions favored by the more reliable models end up trending toward the slower end of the guidance pack with this system...which makes sense given the higher long wave amplitude. As such...the systems departure was slowed in the forecast which resulted in increasing probability of precipitation into Friday. While its departure is slower...much drier air does work into the area by Friday night with the consensus offering extremely lean layer relative humidity profiles and weakening northwest cold advection. This sets the stage for stellar radiational cooling going into Saturday morning. Gridded guidance suggests widespread 30s across the area...which with 850 mb temperatures falling toward -2c is not too shocking. Frost was added to the grid to accommodate lows generally in the middle 30s across vast tracts of the County Warning Area...and frost/freeze headlines seem possible at this point. These temperatures at 850 mb fail to moderate during the day Saturday much...so temperatures will likely only wander for highs from the upper 50s Friday toward the lower 60s Saturday. Fries && Long term /Saturday night through Wednesday/... similar conditions persist through the rest of the weekend and even possibly into Monday morning with very dry conditions and light winds favoring Superior radiational cooling each night. As such...lows continue to look to head toward the 30s each night...with a few locations inevitably dropping below freezing. Once again...frost was added to the forecast into the overnight/early mornings this weekend...and frost/freeze headlines again seem quite possible. Otherwise...very gradual moderation of low level temperatures will be likely as the closed system departs eastward but dry cyclonic northwesterly flow aloft remains the large controlling factor for our weather. This is thanks in part to the fact that the model consensus keeps the large scale ridge axis largely to our west through the middle of next week. As a result...conditions should remain generally dry with very slow moderation of temperatures through the week. Fries && Aviation /21z Wednesday through Monday/... low to medium confidence forecast on shower and thunderstorm timing. VFR weather will prevail much of the forecast period /at least into the overnight hours/...the challenge is timing on any rain showers or thunderstorms and rain impacting the terminals. Maintained thunderstorms in the vicinity for late afternoon and evening before transitioning to VFR rain showers until midnight. Clouds will develop in the predawn hours...latest guidance suggest ceilings in the 015-025 which is higher than earlier runs and in line with ongoing forecast. Stratocu will lift by middle morning into a low end VFR deck. For places that experience rainfall...patchy fog could develop during the predawn hours. 98 Outlook.../Thursday through Sunday/... the probability of widespread IFR weather is low. && Pbz watches/warnings/advisories... Maryland...none. Ohio...none. PA...none. WV...none. && $$ 07