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

Area Forecast Discussion
National Weather Service La Crosse WI
513 PM CST Sat Dec 19 2020 .SHORT TERM...(This evening through Sunday) Issued at 231 PM CST Sat Dec 19 2020 Main forecast concerns in the short-term are on cloud trends and low temperatures tonight. Stratus remains in place across the entire forecast area this afternoon. The back edge, located across south central Minnesota and north central Iowa, was very slowly edging east this afternoon. RAP forecast soundings suggest the stratus will break up and push east tonight. However, not expecting to see much drying out of the stratus layer as there is very little subsidence in place. Thinking the the stratus will gradually push east on the back side of the surface ridge as winds in this stratus layer switch around to the southwest and increase tonight. Clouds will have a huge impact on temperatures tonight. Where clouds clear lows could fall into the mid to upper teens. Where clouds remain, lows will stay in the upper 20s. We should see the clouds clear across southeast Minnesota and northeast Iowa this evening, then after midnight across western into central Wisconsin. The daytime hours on Sunday will feature quiet weather across the area but that changes Sunday night as a fast moving system sweeps across the area with some light snow. High temperatures will be pretty pleasant for mid December on Sunday with highs ranging from the upper 30s over northeast Iowa to the lower 30s across northcentral Wisconsin. .LONG TERM...(Sunday night through Saturday) Issued at 231 PM CST Sat Dec 19 2020 Mid-high level GOES water vapor imagery showing the Sunday weather system now crashing onto the British Columbia coast. Open cellular convection is seen in a wave energy train upstream of the coast with GOES 17 cloud drift winds showing a strong zonal jet of 140-150kts...impressive! This system shifts across the area Sunday night and Monday with descent QG forcing through the troposphere and a warm advective band of precipitation. Models coming more into agreement that snow will fall with this system north of I-94, with confidence decreasing southward. Some 19.12Z model solutions /EC, HiRes Window NMM, SREF mean, etc/ splash very light precipitation southward over most of the area. This is where confidence is lower right now - just how far south will see measurable snowfall of a few tenths of an inch of snow. In the 19.00Z ECMWF ensemble, all 50 members snowed in KRST (all <0.5"), and about 90% in Platteville WI. Light, but a few tenths of an inch of snow. Generally continued the forecast of 1-2" accumulations north of I-94 in northcentral WI (agrees well with 19.12Z HREF accumulations), with less than 0.5" elsewhere. But, have not had the confidence to increase precipitation chances much south of I-90. It may be that almost all areas see very light, measurable snow midnight Sunday to noon Monday. Winds really kick in Monday post-frontally with gusts to 35 mph and possible some snow/rain showers. Above-average temperatures are the story through Wednesday, but the mid-week period is really in question. Model solutions vary wildly from a Plains blizzard to the northwest to a band of snow to the south. Inspection of the Canadian/GFS/ECMWF ensemble families and cluster diagnostics suggests predictability is low for that period and has not generally increased over the past 18 hours. There certainly could be higher impact weather in the area, but then again, maybe not. Thus, it seems most prudent to allow the lower precipitation chance, model blend to ride for now with && .AVIATION...(For the 00Z TAFS through 00Z Sunday evening) Issued at 513 PM CST Sat Dec 19 2020 Stratus clouds with IFR/MVFR ceilings will hang around this evening as the cloud deck slowly clears out from west to east tonight. Some areas of BR will hang around as well with visibilities possibly dipping into 3-5 SM at times. Not fully confident on the clearing times, but was a little more pessimistic and slightly delayed the clearing at KRST. After the clouds clear out around 05z at KRST and 07z at KLSE, VFR conditions are then expected through the remainder of the TAF period. Winds will generally remain around 10 knots or less from the west/southwest. && .ARX WATCHES/WARNINGS/ADVISORIES... WI...None. MN...None. IA...None. && $$ SHORT TERM...Wetenkamp LONG TERM...Baumgardt AVIATION...Peters
Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
337 PM EST Sat Dec 19 2020 .DISCUSSION... Scattered light precipitation was in place across SE MI earlier this morning and afternoon as sufficient moisture ahead of an approaching cold front partially saturated low-levels. Have bumped PoPs down for this afternoon given the drier radar trends and with hourly dprog/dt of hi-res showing a drier 0-4 kft layer relative to early morning runs, but should start to see an uptick in coverage as the cold front and trough axis, in place across western Michigan, starts to sweep through the state. Thermal profiles support all rain (some melting flakes possible at onset) across the Metro region for new precipitation, given the increasing wbz heights and temperatures now in the mid to upper 30s. The Tri-cities and Thumb remain slightly cooler, but an eventual transition to rain is probable as wbz heights gradually increase later this evening. A dry slot is also projected to fill in from south to north through the evening, stripping ice nuclei from the dgz, further supporting an eventual transition to all liquid. Temperatures are expected to remain just above freezing late tonight and through the overnight hours, so any additional snow accumulation will remain limited to elevated and grassy surfaces -- totals under a half-inch, likely confined to the Thumb. Otherwise, patchy fog development will be possible this evening and overnight given dew points hold around 32 degrees. Surface cold front/trough will push across and wash out over SE MI late tonight and overnight with very subtle backing of low-level winds advecting some Lake Michigan moisture into the area. This should help support some drizzle chances and extensive cloud cover during the overnight period. Overnight temperatures will not drop much as a result, holding in the lower 30s. Dry low-level air will gradually fill in behind the front Sunday morning into the afternoon, greatly diminishing PoP chances. There will be modest instability for December standards building up during the late morning and early afternoon, which could help reinvigorate some spotty light showers, however, the low-level environment is dry (0-2 km RH below 60 percent) and convective depths are low (below 5kft). Regardless, the HRRR is hinting that some hydrometeor production could make it down to the surface, so will hold slight PoPs through the afternoon. Saturation falls well below the moisture- starved dgz, so any precipitation observed would fall as rain. All PoP chances to come to an end in the late afternoon and evening as daylight wanes and dry air continues to fill in. Little change in the thermal profile to again support highs in the mid to upper 30s with overnight lows dropping into the 20s. Strong convergence is noted between the NAEFS and EPS ensemble forecast system regarding a clipper system that drops from the northern Plains late Sunday and arrives over Michigan Monday afternoon into Monday evening. Convergence of this system was also noted 24 hours ago across ensemble systems. As the low arrives over Michigan, SE MI will fall on the cyclonically sheared side of a 130 knot jet and will have support of multiple PV waves while westerly flow helps advect moisture off of Lake Michigan. Given the above, have bumped up PoP chances to likely (~60-70 percent) in the Monday afternoon-evening time frame. Temperatures will once again hold in the mid 30s when precipitation is expected, thus ptype will fall between rain/snow or all snow. This low pressure will weaken while approaching Michigan and forcing is not strong, so early accumulation potential will hold to or below an inch. Last, a large amount of variance is still observed between ensembles regarding a late week system expected to impact the Great Lakes region. A strong Pacific wave will crash onshore late Monday into Tuesday which will amplify troughing across the southwest. Cyclogenesis will take place as a result, but large uncertainties exist regarding the potential track of the surface cyclone -- A wide dispersal is noted for individual ensemble members. This trough and associated cold front will eventually impact SE MI later next week, bringing active weather and colder temperatures around the holiday weekend. Additional convergence will be needed between models in order to pin down temperature trends and any snow potential. && .MARINE... Weak system moving across the central Lakes today as a frontal boundary stalls out. This will bring weakening southwest flow into tonight as precipitation lingers over the lakes into Sunday morning. Another system tracking across northern Ontario will bring moderate southwest flow again Sunday afternoon with gusts generally under 25 knots. Progressive pattern continues into early next week as an Alberta clipper is expected to swing across the region Monday and Monday night. The result will be a surge of colder air and increasing northwest flow Monday night with gusts approaching 30 knots. The middle of next week also favors unsettled marine conditions. && .PREV DISCUSSION... Issued at 1247 PM EST Sat Dec 19 2020 AVIATION... Low-level moisture continues to stream into Southeast Michigan early this afternoon with MVFR ceilings filling into the remainder of the terminal corridor over the next several hours. Ascent ahead of the inbound low pressure/cold front and sufficient moisture depth results in additional light precip this afternoon and evening. Thermal profiles, recent obs, and surface temps above freezing suggest a rain/snow mix transitioning to light rain later this afternoon. The exception will be MBS and possibly FNT where light snow may remain dominant. Midlevel moisture strips out of the area later this afternoon, bringing into question the presence of ice nuclei in the cloud layer this evening and especially tonight. As a result, precip will likely change over to drizzle with fog tonight as the low-levels remain saturated under the stalled frontal inversion. IFR ceilings likely develop this evening with a period of LIFR possible late tonight into early Sunday as the front stalls and weakens over the area. Conditions then slowly improve during the daylight hours Sunday. For DTW...Any light precip should fall as a rain/snow mix early this afternoon before light rain generally becomes favored by late afternoon into this evening. Drizzle with fog then becomes most likely tonight as midlevel dry air moves in over saturated low levels. MVFR ceilings will spread in from the northwest over the next 3 to 4 hours with IFR ceilings then becoming likely after 03z. DTW THRESHOLD PROBABILITIES... * High in ceilings below 5000 feet this afternoon through Sunday morning. * Moderate in precip type as a rain/snow mix this afternoon. High in precip type being all rain this evening and overnight. && .DTX WATCHES/WARNINGS/ADVISORIES... MI...NONE. Lake Huron...NONE. Lake St Clair...NONE. Michigan waters of Lake Erie...NONE. && $$ DISCUSSION...AM MARINE.......AA AVIATION.....TF You can obtain your latest National Weather Service forecasts online at
Area Forecast Discussion
National Weather Service Nashville TN
539 PM CST Sat Dec 19 2020 .UPDATE... FOR EVENING DISCUSSION. && .DISCUSSION... Precip in the form of showers has made its way across the western half of the mid state this early evening. OHX sounding release is revealing freezing levels in the 6kt to 7kft range. Latest Hrrr in agreement with the categorical pops spreading from west to east overnight, so fcst on track. Otw, fcst lows are very close to wet bulbs across the area so no need to adjust our mins. Fcst appears to be on track so no changes. && .AVIATION... 00Z TAF DISCUSSION. Rain continues to move in from the west, and cigs will continue to drop this evening and overnight from MVFR to IFR by sunrise. LIFR cigs will also be possible, especially from around 12Z through 16Z or so except for KCSV where LIFR/IFR cigs will be in place for most of the day Sunday. Chances for rain will diminish from west to east starting around 06Z at KCKV, and fog may develop behind the rain by 12Z at KCKV. Cigs should slowly improve during the day for KCKV/KBNA/KMQY and should be VFR by 00Z Monday at those terminals. Winds will generally be light and variable. && .OHX WATCHES/WARNINGS/ADVISORIES... NONE. && $$ DISCUSSION......21 AVIATION........Barnwell