WHAT HAS CHANGED Spatial coverage of showers and storms Wednesday afternoon has shifted a bit farther north. Timing of cold front passage Friday has slowed a bit.

KEY MESSAGES 1) First of two cold fronts this week sweeps through on Wednesday.

2) Stronger late-week cold front returns showers, gusty wind, and temperatures more like that of mid-November.

DISCUSSION KEY MESSAGE 1... The evening features much of the convection that has weakened and slid south of the Mason Dixon Line. Much of the showers will remain in northern West Virginia overnight. A few instances of drizzle will remain north of I-70. Thus, kept some slight chance pops north in to SW PA.

Previous Discussion...A slow moving cold front finally gets a kick and moves through our area north to south this afternoon showers and a few storms. Some storms could get a bit on the stronger side, but the primary limiting factor to that will be a displaced instability axis from the highest shear. The belt of strongest mid-level flow, and thus deep layer shear, will be in the northern half of our area. The best opportunity for heating and better instability will be in the southern half of our area in the thinner cloud cover out ahead of the front. REFS joint probability for >500 J/kg of SBCAPE and >30 knots of shear to capture the best overlap of parameters has shifted a bit farther northwest in several iterations of runs with around 60- 90% from eastern OH, through far southwestern PA, and into northern WV. A handful of recent HRRR and RRFS MPAS runs favor a slightly slower moving front and thus storms firing a bit farther north, which is falling more in line with the REFS shift, so this could be a trend to note through the day today. So, this will be the area to watch for storms to get more feisty and bring a low-end damaging wind threat coincident with SPC's Marginal Risk. Hail seems less likely with thinner CAPE profiles within the hail growth zone and a too shallow equilibrium level capping updrafts before they can maximize on hail growth. With lacking low level lapse rates progged by the latest RAP, we'd likely need to see the deep layer shear organize and congeal the storms into clusters to capitalize on the wind threat. Can't entirely rule out some flash flood concern which would be a bit more concerning if storms do fire farther north and impact southwestern Pennsylvania where two rounds of storms already went through earlier in the morning and preconditioned the ground, but that's still favored in southern OH and northern WV.

KEY MESSAGE 2... The most impactful system arrives on Friday ushering in strong wind gusts, well below normal temperatures, and showers. While some uncertainty remains among ensembles primarily owing to timing and, to a lesser extent, depth of the mid-level trough, confidence in all three impacts remains high. Deep surface low pressure will pull a cold front through the area on Friday with a tight pressure gradient, 10+ dam 500 mb height falls in 12 hours, and strong low- level cold advection.

For the wind threat, the combination of the tight gradient and mixing into a 40-45 knot low level jet should result in widespread gusts of 35-45 mph. They'll be stronger across our higher elevation remains up to ~60%. The already low probability of Advisory criteria in the lowlands has decreased a bit to only around 20-40%, so while still not likely, not entirely out of the question either. Eastern Tucker County runs an elevated probability for High Wind Warning criteria gusts >58 mph also up to ~60%. Wind headlines for the ridges will likely come into consideration within the next 24-36 hours.

Strong cold advection in post-frontal northwest flow will bottom out ensemble mean 850 mb temperatures to around -2 to -4C. An unseasonably cold air mass will settle in for the first half of the weekend providing the potential for two nights of frost and freeze. Friday night, it looks like a freeze or bust as wind will remain elevated in the post-frontal regime. So, while probabilities for <36F lows are high, especially outside of urban areas, not too concerned about widespread frost. Probs for <32F (and an unconditional freeze) sneak up to 40-50% along and north of I-80, so that'll be the area to worry about on Friday night. Saturday daytime highs will feel more like those of mid to late November only warming to near 50 degrees with 40s in the elevation. The gradient will relax some Sunday night allowing wind to slacken and skies will clear. This should point toward Saturday night being the coldest areawide, and possibly the first shot for the Pittsburgh climate site to dip to 32F or lower since May 2nd. With better radiational cooling conditions amid a dry, cold airmass, the dew point could essentially be the floor for lows, and this suggests widespread low to mid 30s and upper 20s. A widespread frost is likely. Probability for a freeze ranges from 40-80% in eastern OH, northern WV, and western PA, with the highest values away from urban areas and in the higher elevation. Expect that we'll be looking at some combination of frost/freeze headlines on top of wind headlines as well.

Two rounds of precip are possible both along the front itself and then in the wrap around cold advection across the lakes on Saturday. Strong flow amid the dynamic system could drive some "power showers" accompanying FROPA on Friday bringing enhanced gusts with them provided they're able to get tall enough to tap into the 40-50 knots a few thousand feet off the ground. This will be dependent on the timing of the front has fluctuated between early morning and early afternoon. Earlier would pose less potential for strong gusts accompanying the showers, later would pose more. It wouldn't require a whole lot of depth to the unstable layer to get things mixed down, so will certainly require monitoring. Then, continued cold advection in northwest flow likely results in some lake effect and upslope precip late Friday into early Saturday. For most, this will be a cool, windy scattered rain/graupel shower setup. For some, the first novelty snowflakes of the season aren't out of the question. Ground temperatures will be too warm to see it stick, and there's some question to the remaining depth of the moisture and whether or not it's deep enough to generate snowflakes or sits in the favored supercooled liquid region. Ensemble 850 mb temperatures suggest the coldest to the north and east with a colder outlier extending farther west. So the most favored area to see some white stuff falling should be the highest elevations of PA and WV and the far reaches of our northern tier of counties, unless the outlier solution comes to fruition and colder air filters in farther west.

AVIATION /00Z THURSDAY THROUGH MONDAY/ The frontal boundary continues to sink southward and has recently passed HLG and LBE, where northerly gusts have picked up. The progress of the front is expected to slow some overnight.

Behind the front, MVFR ceilings continue to drop from north to south. Past midnight, rain coverage will drop off, but ceilings may begin to rise. Models continue to highlight rises back into low end VFR (outside of MGW/FKL/DUJ) with approaching dry air. Ceilings look to be on their way back down to MVFR from south to north tomorrow as the front begins to advance back north as a warm front after sunrise on Thursday. Rain chances through 00Z remain low and preclude mention. Increasing shower coverage is possible after 00Z with increasing warm advection.

Outlook... The warm front should return shower and possible thunderstorm chances later in the day through Thursday night, along with continued ceiling restrictions. A strong cold front is expected to cross the region from W-E Friday, with showers and W-NW wind gusts from 30-40kt. Gusts from 40-45 kt are possible across the higher terrain.

VFR should return Saturday and Sunday as wind diminishes under building high pressure.

PBZ WATCHES/WARNINGS/ADVISORIES PA...None. OH...None. WV...None.

DISCUSSION...MLB/Shallenberger AVIATION...CL/AK

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