Forecast Discussions mentioning any of "HRRR" "RAP" "RUC13" "RUC" "RR" received at GSD on 12/05/20

Area Forecast Discussion
National Weather Service Boston/Norton MA
1026 PM EST Fri Dec 4 2020 .SYNOPSIS... A strong storm system will impact the region Saturday into Saturday night bringing heavy rain and strong winds along the coast, with potential for heavy wet snow and power outages across the interior. The storm exits Sunday, then mainly dry and colder weather Monday and Tuesday with a moderating trend for the middle of next week. && .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/... 1020 PM Update... Forecast is largely on track so only made minor changes to better match the current obs. Our focus tonight will be on fine tuning the details of tomorrow`s winter storm. PREVIOUS DISCUSSION... Initial band of light rain along south coast was slowly lifting north but plenty of dry air at lower levels means it will take its time making progress. Expect light rain to reach Mass Pike corridor around 10 or 11 PM but locations farther north may not see much at all tonight. 22z HRRR seems to have a good handle on timing. By late evening and especially toward morning intensifying low pressure across the mid Atlantic states will be lifting northeast. This increases the forcing for ascent and widespread rain should overspread most of the region by daybreak. Low temps will mainly be in the upper 30s to lower 40s...but it will be a few degrees cooler in the high terrain. && .SHORT TERM /6 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/... Highlights... * Heavy Wet Snow with the potential for numerous power outages and tree damage in the high terrain of central MA * Uncertainty exists in westward extent of heavy snow/power outage risk and Ptype issues across the coastal plain * Strong Winds with Heavy Rain/Street Flooding expected near and southeast of the Boston to Providence corridor Details... Low pressure will undergo bombogenesis as it rapidly lifts northeast from the mid Atlantic and towards the Cape/Islands over the next 24 hours. The surface low drops over 30 MBs as it passes to our east at around 980 mb Saturday evening. If we were dealing with a seasonably cold airmass a blockbuster snowstorm would be in the cards for the most of the region. In contrast, the current airmass in place is quite mild for this time of year with temperatures still in the 50s late this afternoon. That being said, the rapid intensification and strong dynamics of this system will allow for soundings to eventually become isothermal in some locations. As the mid level centers close off and track across the far southeast New England coast, very strong omega of 30 to 50 units will support heavy precipitation. The specific track coupled with very sensitive thermal profiles will make the difference between heavy rain and 1-2 inch per hour snowfall rates. We will break it down a bit more below by region. Central MA... Lets start off where confidence is highest and that is across central MA and especially the Worcester Hills. While rain should be falling Saturday morning, expect very strong dynamics to result in a change over to heavy wet snow during the mid to late morning hours. This will first occur in the highest terrain of central MA and then impact the rest of that region. Given the evolution of the storm and rapidly developing mid level centers, expect snowfall rates of 1 to 2 inches per hour once the transition occurs. Unlike the late October snowstorm...925T will only drop to between 0 and -2C. The result will be a heavy wet snow with temp on the order of 32/33 degrees. That is problematic because we are anticipating 6 to 12 inches of wet snow in this region with the greatest risk of seeing a foot of snow in the highest terrain. In fact, we can not rule out the low risk of 12 to 16 inches of snow. On the flip side of the coin, we want to be careful given marginal thermal profiles. If it ends up a little warmer we could up with more rain than expected and lower snowfall amounts than our current forecast reflects. But putting it all together, we are quite concerned for the potential for significant power outages and some downed trees if the heavy snow amounts are realized. Winter Storm Warnings have been posted for this region. Western MA and northern CT... Despite the event being less than 24 hours away, uncertainty remains larger than normal for western MA and CT. The 12z model suite has trended further west and also a bit milder than the earlier 00z runs. Across western MA and northern CT, the threat exists of rain changing to heavy wet snow but is dependent on track. If track shifts slightly back to the east, the bulk of the heavy snow would remain east of the CT River Valley. The limited amount of dynamics/intensity would also create Ptype issues especially in the lower elevations of the CT River valley. Based on the westward shift, we did opt to include Winter Storm Watches into Tolland and Windham counties for the potential of heavy wet snow of 6+ inches and scattered power outages. The 18z NAM and latest HRRR runs do show some concern for Hartford county too, so something will have to watch closely. We also left the remainder of western MA is the Watch as a small change in track will make a big difference. The 00z model runs this evening will shed more light on the situation. Eastern MA/RI... Finally, there is an unusual amount of uncertainty across eastern MA/RI inside 24 hours. While heavy precipitation is pretty much a lock, the westward shift in the 12z model guidance has resulted in a somewhat warmer solution. It probably will take until mid to late afternoon for rain to change to snow across this region. Still the potential for warning level snows in metrowest with snow accumulations into the Boston to Providence late Saturday into Saturday evening as well. Greatest concern for heavy wet snow and power outage risk will be across interior northeast MA, where a Winter Storm Watch remains in place. In fact, we can not rule out heavy snow/warning criteria in parts of northwest RI if the track shifts a bit east. Winter Weather Advisories may eventually need to be extended further southeast. Heavy Rain/Strong Winds... Ptype will mainly be rain near and especially southeast of the Boston to Providence corridor. In fact...heavy rain on the order of 2 to 3 inches with some locally higher amounts may result in pockets of urban and poor drainage street flooding. If 3+ inch amounts are realized a few mainstem rivers and small streams may experience minor flooding. Lastly, a high wind warning has been issued for coastal Essex County as well as coastal Plymouth County, Cape Cod and the Islands. A very strong pressure rise/fall couplet in excess of 10 MB in 3 hours will bring NNW wind gusts of 50 to 60 mph late Sat into Sat evening. We may have to issue wind advisories further west for a period of 40 to 50 mph wind gusts. && .LONG TERM /SUNDAY THROUGH FRIDAY/... 200 PM Update: Fairly quiet weather expected through the period. We are watching another potential cutoff low from the Ohio Valley to the mid Atlantic states early next week, but with lack of downstream blocking this time (i.e., no ridging over Greenland) system is likely to pass well south of New England. Did include some chance PoPs near South Coast, Cape, and Islands but for most part expect this system to have little impact on SNE (as of right now, anyway). Upper flow turns more zonal for mid to late week which should allow for temperatures to moderate into 40s to around 50. && .AVIATION /04Z SATURDAY THROUGH WEDNESDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Tonight through Saturday night...Moderate Confidence. VFR most of the night but expect conditions to lower to MVFR near south coast overnight and rest of SNE by daybreak Sat, before lowering to IFR Sat morning areawide. Rain changes to snow Sat, first near Berkshires and ORH by late morning (14-16Z) where 1/4-1/2SM likely and snowfall rates of 1-2"/hour most of afternoon. Tougher call from BDL-PVD-BOS but thinking changeover occurs early afternoon at BDL (18z) then at BOS/PVD later in afternoon (20-22Z). All rain for Cape/Islands. Conditions improve quickly Sat evening to VFR, mainly 02-05Z Sun. Strongest winds expected later Sat afternoon and evening with NW winds gusting to 25-35 kt inland and 40-50 kt along the coast with 50 kt gusts most likely on Cape Cod/Islands. KBOS TAF...Moderate confidence in the TAF. KBDL TAF...Moderate confidence in the TAF. Outlook /Sunday through Wednesday/... Sunday: VFR. Windy with gusts up to 35 kt. Sunday Night: VFR. Breezy. Monday: VFR. Monday Night through Tuesday Night: VFR. Breezy. Wednesday: VFR. && .MARINE... * Storm Warnings Saturday Afternoon and Evening * Tonight through Saturday night... A significant Marine event will occur Saturday afternoon and night as low pressure undergoes bombogenesis and tracks across the Cape/Islands and moves NE from there. The strongest winds will be from the N and then NNW as the storm rapidly intensifies resulting in 45 to 50 knot wind gusts. Strong Gales are in effect across western sounds, Narr Bay and Boston Harbor with Storm Warnings for the rest of our waters. 15+ foot seas expected across our outer most waters. Outlook /Sunday through Wednesday/... Sunday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Rough seas up to 14 ft. Sunday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of rough seas. Monday: Winds less than 25 kt. Areas of seas approaching 5 ft. Monday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Slight chance of rain. Tuesday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of rough seas. Slight chance of rain. Tuesday Night through Wednesday: Winds less than 25 kt. Local rough seas. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...Winter Storm Watch from 7 AM EST Saturday through Sunday morning for CTZ003-004. MA...High Wind Warning from 1 PM Saturday to midnight EST Saturday night for MAZ007-019-022>024. Winter Storm Watch from 7 AM EST Saturday through Sunday morning for MAZ002-003-005-006-008>011-014. Winter Storm Warning from 7 AM Saturday to 7 AM EST Sunday for MAZ004-012-026. RI...None. MARINE...Storm Warning from 1 PM Saturday to 3 AM EST Sunday for ANZ231>234-250-251-254>256. Gale Warning from 10 AM Saturday to 4 AM EST Sunday for ANZ230- 236. Gale Warning from 10 AM Saturday to 7 AM EST Sunday for ANZ235- 237. && $$ SYNOPSIS...Frank/JWD NEAR TERM...Frank/Chai/JWD SHORT TERM...Frank LONG TERM...JWD AVIATION...Frank/JWD MARINE...Frank/JWD
Area Forecast Discussion
National Weather Service Lincoln IL
839 PM CST Fri Dec 4 2020 .SYNOPSIS... Issued at 318 PM CST Fri Dec 4 2020 Mild and dry weather is on tap tonight through Saturday...before an upper-level disturbance brings a few sprinkles or snow flurries to central Illinois on Sunday. && .UPDATE... Issued at 839 PM CST Fri Dec 4 2020 Skies are clear and winds are variable. Temps have dropped off quite a bit already, but probably will not drop much more. Current overnight low forecast of the middle 20s still looks good, given current dewpoints in the middle to upper 20s over most of the area and lower 30s in the southeast. Lower clouds seen on satellite loops coming off Lake Mich may move into northeastern parts of the CWA as winds overnight shift to north to northeast, but they probably wont get here until toward morning. Current forecast already showing some increasing cloud cover then. Since current forecast looks good, no update planned at this time. && .SHORT TERM...(This evening through Saturday) ISSUED AT 318 PM CST Fri Dec 4 2020 21z/3pm surface analysis shows a weak cold front extending from central Michigan to southern Minnesota. This boundary will drop southward through central Illinois this evening, resulting in winds veering from W/SW to N/NE after midnight. With deep-layer moisture absent, no cloud cover/precip will occur with FROPA. Once the boundary layer flow becomes northeasterly, lake-enhanced moisture from Lake Michigan will begin to spill into east-central Illinois overnight. RAP 900-950mb layer RH field continues to suggest the NE KILX CWA around Champaign and Danville will become overcast toward dawn. Meanwhile further west, skies will remain mostly clear. Low temperatures will drop into the middle 20s. The lake-enhanced cloud band will linger along/east of I-57 through Saturday morning before gradually shifting eastward into Indiana by afternoon. Given E/NE winds, highs will be a few degrees cooler than today...generally in the lower to middle 40s. .LONG TERM...(Saturday night through Friday) ISSUED AT 318 PM CST Fri Dec 4 2020 The next weather system of interest is currently evident on water vapor imagery over Saskatchewan. As this short-wave trough dives southeastward, all models are indicating that it will amplify into a closed 500mb low on Saturday...with the low then tracking across central Illinois during the day Sunday. Despite a lack of moisture to work with, think synoptic forcing will be strong enough to warrant a chance for sprinkles/flurries. Have added flurries to the forecast Saturday night across the NW CWA,then everywhere by Sunday morning. With high temperatures reaching the lower 40s, any lingering light precip will change to sprinkles Sunday afternoon. A second amplifying short-wave will quickly follow in its footsteps, carving out a deep long-wave trough across the Great Lakes/Ohio River Valley by early next week. With the second wave passing through the region, have continued the chance for flurries Sunday night into Monday before synoptic subsidence in its wake brings clearing skies across the board by Monday afternoon. After that, the deep trough will shift eastward...allowing upper heights to rise substantially across the Midwest by the middle and end of next week. As a result, high temperatures will warm into the lower 50s by Wednesday/Thursday. && .AVIATION...(For the 00Z TAFS through 00Z Saturday evening) Issued at 535 PM CST Fri Dec 4 2020 Weak cold front currently across northern Indiana/Illinois will sink southward tonight...resulting in winds veering to the N/NE overnight. Low clouds over Lake Michigan will gradually advect S/SW tonight and will spill into east-central Illinois toward dawn. Based on RUC 900-950mb layer RH field, it appears KCMI will be on the back edge of the lake-enhanced cloud band. Have therefore maintained a period of MVFR ceilings between 11z and 15z at KCMI, with only SCT low clouds at KBMI/KDEC. Despite relatively dry airmass, several CAMs suggest patchy fog late tonight. Since fog formed last night, went with a persistence forecast and brought visbys down to around 4 miles at all sites after 10z. && .ILX WATCHES/WARNINGS/ADVISORIES... NONE. && $$ UPDATE...Auten SYNOPSIS...Barnes SHORT TERM...Barnes LONG TERM...Barnes AVIATION...Barnes