Forecast Discussions mentioning any of "HRRR" "RAP" "RUC13" "RUC" "RR" received at GSD on 12/20/19

National Weather Service Wilmington OH
946 PM EST Thu Dec 19 2019 .SYNOPSIS... Cold high pressure will continue to control regional weather this evening through the overnight before giving way to warming temperatures as the high exits further to the east and upper level ridging begins to working in from the southwest. Conditions will be mostly dry through the period. && .NEAR TERM /THROUGH FRIDAY/... It is shaping up to be a very quiet overnight period across the ILN FA with broad surface high pressure entrenched across the eastern Ohio Valley. It will also be another cold night, although not as cold as last night -- as lows will bottom out in the lower 20s with the exception being where a more substantial snowpack still resides from east-central IN through central Ohio where some mid teens will be possible by daybreak. Although there were some hints at the potential for some patchy lower clouds or even some light fog to develop, confidence is not overly high in this materializing and therefore have kept the FA mainly clear through daybreak. Winds will be light and variable with perhaps a light southerly component at times. ...PREVIOUS DISCUSSION... Clear skies will continue for most areas this evening through early Friday morning. There is some potential for increasing low clouds and some fog to form toward daybreak north of Interstate 70 as a weak cold front heads east through the Great Lakes. This showed up particularly well on the HREF and RAP earlier this morning, and has now appeared on the 12Z NAM and GFS as well. && .SHORT TERM /FRIDAY NIGHT/... On Friday, the weak boundary shifts to the east, leaving more warm air advection regionwide. However, where snowpack and low clouds linger across the north, highs will struggle to reach the mid 30s. Elsewhere, highs will approach normal mid-December values. Friday night, continuing light southerly winds will keep lows in the mid 20s to around 30, or slightly above normal. && .LONG TERM /SATURDAY THROUGH THURSDAY/... High confidence in a quiet period with temperatures averaging out above normal, and without hazards from precipitation or wind Saturday through Wednesday. Lower confidence creeps into the forecast on Day 7 /Thursday/ with regards to rain chances. Saturday-Wednesday - focus this period will be on upper trough and closed low which will be in the Ark-La-Tex region on Saturday morning. This system will move slowly east-southeast at it digs into/along the Gulf Coast region by Sunday morning and then ejecting east-northeast off the Georgia coast on Monday. There has been slow model-creep northward in the past few days on the northern fringe of light rain potential attendant to this system - from far southern Tennessee several days ago into southern KY in recent runs. And while the 19.12Z GEPS probabilities suggest some low-end rain chances making it as far north as the Ohio River on Sunday night or Monday morning, just not seeing similar agreement in 19.00Z EPS nor 19.12Z GEFS probabilities, nor are we seeing anything in the higher- resolution deterministic or control runs for these ensembles. So will be keeping the forecast area dry through the weekend with only some increase in clouds as the cirrus shield tries to cover the south/east portions of the forecast area on Sunday/Sunday night. To support this, EPS/GEFS/GEPS ensemble mean PWATs are all well below seasonal normals through this period in the subsidence/ridging region on the north/west side of the cyclone. On Tuesday, upper trough will shift into the Intermountain West which forces more substantial and full-latitude ridging over the eastern US. This will keep dry conditions over the forecast area Tuesday through Wednesday. There`s pretty solid agreement /for Day 7 in December/ of energy ejecting out of the Desert Southwest and up the western side of the southeast US ridge and then topping the ridge through the Great Lakes on Thursday. All deterministic runs of GFS/CMC/ECMWF bring a weak cold front through the Ohio Valley sometime either Wed night or Thursday. Rain chances in question - 19.00Z EPS has very low probabilities for measurable precipitation, with 19.12Z GEPS in the 20-40% range, and most of the GEFS members showing a swath of light rain along the front. With NBM giving 20- 30% chances, was inclined to keep those for now as there`s enough there to mention some small rain chances. Of concern is a PWAT anomaly that is drying with time as the system crosses the Ohio Valley, and a parent shortwave well to the north of the area, relying only on frontal convergence to generate some rain, not to mention the offshore trajectories in the Gulf for a while early next week in the wake of the departing southeast US upper trough which will delay moisture return to the system. Temperatures will incrementally rise Saturday through Tuesday as +2 sigma 500mb height anomalies combine with 850mb temp anomalies slowly climbing toward 1.5 sigma. Lower levels will be slower to warm given light winds / weak mixing through the length of period - not to mention seasonally low sun angle. Also evident in temp forecast - is residual 2-5" snow pack from southeast IND into west- central OH which will be slow to erode given factors mentioned above. While ECMWF ensemble suggests it may take 4-5 days to erode this snow pack, it may be a little faster than that suggests. However, snow pack will impact temperatures across the northern CWA at least until Monday/Tuesday, keeping them in the 40s. As clouds thicken and front approaches on Thursday, kept max temps in check due to expected lack of sunshine - but still well above normal. && .AVIATION /02Z FRIDAY THROUGH TUESDAY/... VFR conditions are expected through the entire period, although there continues to be a hint of some very thin low level moisture developing into northern portions of the region later tonight into Friday morning. This could manifest itself in a FEW clouds below 500ft or possibly some brief VSBY reductions, but confidence is not overly high on either of these conditions being met at even the northern sites. Did try to scale back a bit in ongoing fcst but did not feel confident enough to yank altogether. As such, have included a TEMPO group for KDAY, KCMH, and KLCK for a few clouds in the 300-500ft range in the several hour period around sunrise with some brief minor VSBY reductions. However, this may even be overplayed a bit but trends will undoubtedly be watched as we progress through the overnight period. Otherwise, light and variable winds with a gradual increase in cirrus will be the main story for the period. Could very well see the same low level moisture develop again Saturday morning but will omit from the outlook for now as confidence remains low in such an occurrence. OUTLOOK...No significant hazards are expected at this time. && .ILN WATCHES/WARNINGS/ADVISORIES... OH...None. KY...None. IN...None. && $$ SYNOPSIS...Hogue NEAR TERM...KC/Hogue SHORT TERM...Hogue LONG TERM...Binau AVIATION...KC
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
846 PM CST Thu Dec 19 2019 .UPDATE... 845 PM CST Only minor updates this evening were to nudge low temperatures up ever so slightly and to refine the near-term patchy fog wording in the gridded forecast. The surface gradient continues to slowly relax this evening with a col/saddle point noted in the surface flow just north of Waukegan at this hour where a weak/diffuse backdoor boundary has slipped southward down the lake. To the southwest of this boundary, southwest to west-southwesterly flow in the low-levels is promoting some modest warm advection ahead of a shortwave pivoting towards western Minnesota. This slightly warmer airmass will continue to inch across our region tonight which could result in the development of some fog through the rest of the evening hours. Already seeing some 5-6 mile reports at this hour, although the combination of multi-degree F dewpoint depressions, increasing high cirrus, and a very thin near-surface hydrolapse coupled with a limited snowpack don`t seem to portend a widespread dense fog potential. That said, the inherited area-wide patchy fog wording tonight seems reasonable, with perhaps a corridor more favored for some isolated pockets of lower visibilities north and east of a Rockford to Evanston line where lowest-100 mb winds will be lightest and the near-surface hydrolapse will be greatest. Updated products have been transmitted. Carlaw && .SHORT TERM... 223 PM CST Through Friday... Only forecast concern is the potential for fog tonight into early Friday. Temperatures have climbed back above freezing after a mid week cold spell. High clouds continue to spread out ahead of an upper trough back across the Rockies and up atop an upper level ridge across the plains into our area. The ridge axis will shift overhead tonight which may keep the eastward extent of the clouds at bay. The ridge will then spread to the eastern Great Lakes Friday. At the surface, high pressure will remain parked across the eastern USA, with a weaker low out ahead of the aforementioned upper trough. The surface pressure gradient will temporarily weaken tonight as the upper ridge passes overhead and with the surface high to the east, southwest flow will usher in a bit more lower level moisture and relatively warmer air over our cool and still partly snow covered air mass. While this is a generally favorable pattern for fog/low cloud development, the moisture advection is not all that strong in this setup nor is the snowpack very thick or widespread. Guidance remains mixed as to the degree of lower clouds and fog, including the HRRR and the RAP which are not all that excited, but there could be some localized pockets of thicker fog. The high cloud cover may also act to insolate areas as well and mitigate this too. Thus we continue with a light fog mention for the time being. Certainly if clouds are not too extensive we would add at least few degrees to highs on Friday. The low level inversion is stout on Friday but shallow, thus we should be able to break anything up that may form. Expect high clouds will thicken a bit more as the upper trough approaches Friday afternoon. KMD && .LONG TERM... 245 PM CST Friday Night through Thursday... An extended period of mild and dry weather is expected to continue through Christmas as broad, slowly progressive upper level ridging moves across the central CONUS. Latest guidance continues to support the warmest period to be Sunday through Wednesday. While the latest model runs have backed off a little on max temps through this period, with highs in the middle to upper 40s. However, the GFS/ECMWF continue to indicate 925 mb temps of 10-12C for Monday, which, climatologically, would support temperatures in the low 50s. So, there is still the potential to see Max T on Monday in the low 50s should cloud cover be low enough to maximize insolation. While mild weather will continue through mid-week, conditions should be slightly cooler, with more extensive cloud cover and the passage of a weak surface cold front whose associated upper system is of Pacific origin. Unfortunately model solutions begin to diverge significantly with the timing and strength of this system, so while there is relatively high confidence in temperatures remaining above normal, the timing differences introduce greater uncertainty to the forecast by Wednesday, and especially beyond Wednesday, with regard to pcpn chances. The GFS/ECMWF both are beginning to introduce pcpn into the forecast for Tuesday and Wednesday, but the GFS is at least 18 hours faster than the ECMWF and the ECMWF develops a much stronger mid-level shortwave moving across the region. So, have kept PoPs at slight chance levels due to the uncertainty. The one item of greater confidence is with thermal profiles, which should remain high enough that any pcpn would be liquid, should there even be any pcpn. The message regarding chances for a white Christmas for nrn IL/nwrn IN remains consistent. Chances for a white Christmas are about as close to zero as possible without categorically stating that there is a zero chance for a white Christmas. Statistically speaking, with regard to forecasts of anything, weather or otherwise, there is never an absolutely zero chance of something happening or not happening. Krein && .AVIATION... For the 00Z TAFs... Main concern through the TAF period remains the potential for MVFR/IFR fog and/or stratus development by early Friday morning. Confidence in this scenario remains low. Winds continue to diminish this evening as the pressure gradient weakens and higher pressure builds in from the east. Winds will be predominately southerly through the period but there could be a brief time where winds turn south southeasterly, although still expect them to remain light. Fog/stratus development is possible by early Friday morning (6-10Z period). Light southwesterly flow in the low levels will help advect at least some moisture into the area and with light surface winds in place this is typically favorable for fog development. However, an area of cirrus is moving in from the west which could limit our surface cooling overnight somewhat which could reduce our potential for dense fog. There continues to be low confidence in how this will ultimately evolve. Have maintained MVFR visibilities but will continue to monitor for the potential for IFR vis/cigs. Our northern TAF sites look to have the best potential for fog based on the latest guidance so have included a mention of 3SM to hint at this trend. Petr && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ VISIT US AT HTTP://WEATHER.GOV/CHICAGO (ALL LOWERCASE) FOLLOW US ON FACEBOOK...TWITTER...AND YOUTUBE AT: WWW.FACEBOOK.COM/NWSCHICAGO WWW.TWITTER.COM/NWSCHICAGO WWW.YOUTUBE.COM/NWSCHICAGO
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