Forecast Discussions mentioning any of "HRRR" "RAP" "RUC13" "RUC" "RR" received at GSD on 02/06/20


Area Forecast Discussion
National Weather Service Des Moines IA
537 PM CST Wed Feb 5 2020 .DISCUSSION.../Tonight through Wednesday/ Issued at 305 PM CST Wed Feb 5 2020 .Overview...Outside of some flurries down toward Bloomfield this evening, expect the next 24 hours to be dry. Light snow is expected to pass over Iowa Thursday night into Friday with another round of snow focused more over northern Iowa and points north Saturday night into Sunday. Below to near normal temperatures are expected, though near to slightly above normal temperatures are generally expected Sunday into early next week. .Details...Longwave trough is slowly moving east of the Rockies this afternoon with southwest flow in the mid and upper levels persisting over the region. Similar to yesterday, low level winds are from the northeast and continue to bring drier air into the state. 12z RAOB from DVN showed a 2.5km layer of dry air between the surface and 725mb while the dry layer on OAX`s RAOB was much shallower with light snow in the vicinity of the balloon`s release. While there are once again radar echos to around 2500 feet, this dry air in the low levels continues to keep evaporating any snow that may try to fall over southern Iowa. HRRR and RAP forecast soundings show this dry layer eroding to some degree this evening, but questionable that full saturation of the column is achieved. Therefore, expect mainly flurries--perhaps a brief period of light snow--if anything over the far southeast part of our forecast area. Some subsidence moves over the state early Thursday and provides for a passing period of sunshine before clouds return as the next shortwave trough digs on the backside of the longwave trough. There is weak QG convergence that will pass over the state Thursday night into Friday with model cross sections showing the low and mid-levels saturating. There is not an overall abundance of moisture to work with to generate much for QPF and therefore snow totals will be on the light side and generally less than an inch. Northwest flow aloft will transition to more zonal flow over the weekend with the next shortwave trough advancing eastward from the northern Rockies. At the surface, low pressure will form and pass over northern Iowa. While this will draw some milder air into the state late this weekend, it will also provide for decent low level thermal lift in concert with fairly robust QG convergence aloft. dprog/dt of the GFS/ECMWF/National Blend of Models has been fairly consistent with the placement of the QPF with the highest values north of Iowa. Current expectation is that there will be light snow accumulations over northern Iowa late Saturday night into Sunday. Into the middle part of next week, a cut off low over the southwestern US will lift over the region during this time period. A period of wintry precipitation, but details are still to be worked out as far as timing and placement. && .AVIATION.../For the 00Z TAFS through 00Z Thursday evening/ Issued at 537 PM CST Wed Feb 5 2020 Widespread VFR conditions expected for the duration of the forecast with mid to high ceilings in far southern Iowa overnight. Surface winds will be light for much of the time with unrestricted visibilities. && .DMX WATCHES/WARNINGS/ADVISORIES...NONE. && $$ DISCUSSION...Ansorge AVIATION...Cogil
Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
948 PM EST Wed Feb 5 2020 .UPDATE... The strong surge of leading isentropic ascent moved east of the region entering late evening after producing snow accumulation generally around 1 inch south of the M-59 corridor. The next phase of the event brings a round of similarly stronger forcing associated with a small but intense mid level wave within the deep SW flow that maintains a background of weak isentropic lift. The associated lift is enhanced within a deep layer of low static stability above 850 mb that has pockets of instability centered around 600 mb and is being fed by specific humidity around 4 g/kg. Coverage may not be as solid as the early evening surge but areas of snow showers increasing again from the SW are expected to be equally productive. An additional 1 to 2 inches of accumulation is likely in this phase of the event between about 10 PM and 4 AM. The Winter Weather Advisory remains in effect with the update including some timing detail going through the late night. Some dry air moves into the area toward sunrise mainly south of I-69 as the mid level wave exits eastward and as the pattern transitions into a band of mid level deformation centered toward the Tri Cities and northern Thumb. This reinforces a diminishing trend in snow activity over the Advisory area after 4 AM, however the deeper dry air intrusion observed in late evening satellite imagery is shown to slide mainly south of the Ohio border. Model soundings also indicate less commitment to a convincing amount of dry air to completely transition to freezing drizzle. Instead expect wavering between precipitation types with no impact as any freezing drizzle or flurries fall on the fresh snow and wet/treated roads through the morning. && .PREV DISCUSSION... Issued at 657 PM EST Wed Feb 5 2020 AVIATION... A band of snow crossed the Indiana/Michigan border around 21Z/5PM and is just beginning in the PTK to DTW area at press time. It brings a trend into IFR while filling in quickly with intervals of LIFR visibility while the band remains at peak intensity during the mid to late evening. A punch of mid level dry air moves in shortly after midnight which reduces the intensity of snow and/or converts it to freezing drizzle. Visibility improvement is then expected while ceiling remains firmly MVFR through sunrise Thursday. The MBS area is favored for persistence of light snow during the morning into Thursday afternoon with some southward expansion toward DTW likely. At this point, the snow combines with stratus to maintain MVFR into Wednesday evening. For DTW... Snow begins and quickly increases intensity during early to mid evening. This brings IFR/LIFR visibility at or shortly after forecast issuance with the first inch of accumulation likely by about 03Z/10PM. The event peak lasts until about 06Z and then diminishes to light snow and/or light freezing drizzle through the late night. MVFR ceiling is expected to hold firm through Thursday morning. DTW THRESHOLD PROBABILITIES... * High for ceiling 5000 ft or less tonight through Thursday. * High for precipitation type as snow this evening into the late night then moderate for light snow and low for mixed freezing drizzle late tonight. PREV DISCUSSION... Issued at 347 PM EST Wed Feb 5 2020 DISCUSSION... Radar echoes are expanding in coverage over Illinois and northwest Indiana in response to strengthening isentropic ascent and a transient period of enhanced right entrance forcing which has been particularly effective in allowing snowfall to accelerate eastward early this afternoon. Increasingly organized radar presentation is accompanied by an expanding coverage of 1/2SM vsbys over west- central IL at press time. This activity will continue to expand eastward in response to eastward migration of the upper jet axis over the northern Great Lakes and will likely reach Washtenaw/Lenawee Counties around 00z, a bit earlier than anticipated. This will add a premium of a few tenths of an inch to the forecast south of M-59, particularly given that the column will still be on the colder side early which will greatly limit riming potential and serve to initially enhance ratios. The 00z to 06z period will be characterized by deepening ascent in the 282-314k layer with a considerable system relative addition to the overall vertical motion field courtesy of the flatter/faster- moving nature of the wave. Meteorological reasoning remains in tact, for the most part, as the the more impactful portion of this event is forecast to occur in response to the moisture-laden (4+g/kg) H85- H7 airmass riding up resident isentropes. Much like yesterday, model cross-sections continue to indicate very low stability within the overrunning airmass with some pockets of potential instability also evident. The chief concern noted yesterday was the potential for any higher intensity snowfall/shallow convection to organize which would in turn warrant an uptick in forecast snow totals. The consensus among the 12z ECMWF/18z NAM/and recent HRRR runs has been that, to varying degrees, sufficient convergence will in fact exist along the 850mb front to sustain heavier snow rates for several hours. Also worth noting that modest overlake instability within northeast flow over Lake Huron may help take the edge off the tight snow gradient north of I-69 by assisting boundary layer saturation there. With this in mind, took a blended approach of the preferred guidance resulting in a band of greater QPF placed across the Ann Arbor/Detroit areas and and a northward expansion to the light snow area. Precip may end as fzdz which will have little or no impact falling into existing snowpack. Finally, snow ratios will generally diminish during the event. First, they will diminish as a result of the aforementioned mid- level warm advection introducing a considerable -5C to 0C mid-level riming layer. Secondly, they will quickly crater after the event peak (after about ~06z) as a result of the highly elevated nature of the DGZ above 12kft AGL. This will be the first layer to get scoured out as the dry slot surges into the the area. Deep saturation and strong lift with pockets of enhanced vertical motion during the event peak will therefore be countered to some extent by the aforementioned tendency for riming. As a result, expect a transition from higher ratio snow / 14:1 / at onset to something closer to 10- 12:1 within the heaviest snow, and then rapidly falling well below 10:1 thereafter. Overall, the event ceiling of around 3 inches remains in tact. Primary forecast adjustment was to raise the floor from 1" to 2" south of M-59 and shift timing forward a bit. Advisory in effect from 8pm - 4am, though the meaningful portion of the event is expected to occur almost entirely in the 8pm to midnight window. Will continue to advertise Thursday into Thursday night as a grind of low intensity snowfall first owing to the approach of the upper trough axis on Thursday afternoon and then a potential glancing shot from the expansive deformation zone extending westward from a developing East Coast cyclone. Certainly plausible to nickle & dime our way to another inch or even two during this period. Guidance suggests temps remain firmly in the 1 sigma range for the next 7 days. MARINE... A low pressure system will track up the Ohio Valley this evening and tonight, increasing the northeasterly gradient over the region. Winds will increase to 15 to 20 kt with gusts to 25 kt tonight into Thursday morning. Wave action /2 to 4 ft/ is currently expected to stay just below criteria for Small Craft Advisories through much of Thursday, but will be monitoring the nearshore marine zones along the eastern Thumb as there will be potential for issuance in this region by Thursday evening. Light to moderate snow is expected this evening and tonight for southern/central Lake Huron, Lake St. Clair, and western Lake Erie as the low passes just to the south. Winds remain out of the north/northeast into Friday as a series of low pressure waves take a similar track up the Ohio Valley, with waves again building over the southern portion of Lake Huron. Diffuse high pressure then takes over for late Friday into Saturday, reducing winds and waves. Scattered snow showers remain in the forecast through early Saturday. && .DTX WATCHES/WARNINGS/ADVISORIES... MI...Winter Weather Advisory until 4 AM EST Thursday for MIZ068>070-075- 076-082-083. Lakeshore Flood Warning until 4 PM EST Friday for MIZ083. Lakeshore Flood Advisory until 4 PM EST Friday for MIZ076. Lake Huron...NONE. Lake St Clair...NONE. Michigan waters of Lake Erie...NONE. && $$ UPDATE.......BT AVIATION.....BT DISCUSSION...JVC MARINE.......TF You can obtain your latest National Weather Service forecasts online at www.weather.gov/detroit.
Area Forecast Discussion
National Weather Service Lincoln IL
917 PM CST Wed Feb 5 2020 .SYNOPSIS... Issued at 315 PM CST Wed Feb 5 2020 Wintry weather will continue tonight, as low pressure passes by to the south of Illinois. A band of 1 to 4 inches of snow centered from southwest Missouri, into central Illinois, then into northwestern Indiana is expected. Areas closer to Interstate 70 will see a mixture of freezing rain, sleet and snow. Light snow may linger through the day on Thursday, but little additional accumulation is expected. Extensive cloudiness will linger into the weekend. && .UPDATE... Issued at 917 PM CST Wed Feb 5 2020 Another lull in precipitation is developing across central IL this evening, with a corridor of spotty precipitation reports stretching all the way back into Oklahoma. Have therefore not changed precipitation chances much through tonight as periods of light snow should continue from Decatur to Champaign westward, while some freezing rain and drizzle mix in to the southeast. Additional snow accumulations to around an inch west of Decatur- Champaign and 1/2 inch or less to the southeast still look good. Evening observations show more coverage of freezing rain/drizzle than previously forecast, as far northwest as Decatur/Champaign, likely due to loss of ice aloft, so have made updates accordingly. Otherwise, temperatures should only drop a degree or two overnight, reaching lows of mid 20s to as high as upper 20s/lower 30s from I-70 southward. && .SHORT TERM...(This evening through Thursday) ISSUED AT 315 PM CST Wed Feb 5 2020 The 12z model runs continue to depict a narrow axis of 2 to 4 inches of snow accum from this system. The 12z NAM, GFS and ECMWF have shown a slight shift northward with the band, and a sharp cut-off on the on southern edge, roughly from Springfield to Danville. However, radar and satellite trends are supporting our current forecast location of the central axis of the heavier snow band extending farther to the southeast toward Taylorville to Paris. South of that line, a surge of warm air will change the precip type to a wintry mix of freezing rain and snow/sleet for a time this evening. Struggled with how far north to take the freezing rain chances, but sided with the forecast soundings in the high res guidance and ended up limiting the northward extent of the wintry mix to Shelbyville to Mattoon. The main change with the afternoon update was to remove our far southern 3 counties from the winter weather advisory, due to limited wintry impacts expected. There may be a period of light snow later tonight into Thursday, but limited icing from freezing rain is expected in those 3 counties (Clay, Richland, Lawrence) overnight. The 18z NAM may not be far off with it`s early ending to the precipitation, as the latest radar mosaic imagery is showing the back edge of moderate to heavy snowfall almost reaching the Mississippi River in W-SW Illinois at 3 pm. Snowfall rates of an inch an hour are likely in the 1/4 to 1/2 mile visibility observations being reported in the heavy snow band, so we have a chance to quickly accumulate into the 1" to 4" range. Therefore, will not make any other changes to the winter weather advisory, other than remove the mention of isolated 5 inch amounts and widen the snow accum range to 1 to 4". We may be able to cancel some of the counties before Noon tomorrow, depending on how active the trailing trough becomes tomorrow. A steady northeast flow will continue tomorrow as a trailing trough of low pressure passes through the area on Thursday. That will keep light snow or flurries lingering during the day. The HRRR is actually showing some Lake Effect snow showers progressing into McLean and Champaign counties Thursday afternoon into evening, but little to no snow accum is expected from that feature. .LONG TERM...(Thursday night through Wednesday) ISSUED AT 315 PM CST Wed Feb 5 2020 A weak shortwave in the southern stream is projected to pass near southeast Illinois Thursday night, triggering a band of light snowfall. That snow could affect our far southeast counties with some light snow accumulation, as indicated in the NAM, GFS and Canadian. Have included some slight chance PoPs in our E-SE counties to account for that possibility. Attention then turns to a fast moving weak clipper moving into our NW counties Friday evening. The model blend solution was washing out that feature almost entirely, but after coordination with surrounding offices, have added chance PoPs for light snow Friday night. The area of interest is includes mainly from NW to SE across our central and SW CWA. Champaign and Danville will be minimally effected by that clipper, based on the 12z models. Cloudy skies and cool west winds will keep temps below normal on Saturday. Then a warm-up is depicted for Sunday as the next low pressure center progresses from the central Plains to SW Wisconsin. Rain chances will increase Sunday afternoon and as the warm front lifts through the area. Rain chances will continue Sunday night as the cold front moves from west to east through Illinois. Beyond that, the upper level flows will be either zonal or favor ridging the first half of next week. Temperatures look to remain slightly above normal from Sunday through Wednesday. However, there will be periodic rain/snow chances, especially Tuesday night per the GFS, and Wednesday per the ECMWF and Canadian. && .AVIATION...(For the 00Z TAFS through 00Z Thursday evening) Issued at 721 PM CST Wed Feb 5 2020 IFR to lower MVFR cigs will be common over central IL through much of the upcoming 24 hours, while vsbys vary considerably from IFR to briefly lower, up to VFR depending on variable precipitation intensity as a showery band of snow continues to stream through central and west central IL. Some patchy freezing drizzle is possible east of this band, and could affect KDEC-KCMI at times this evening. Expect the main snow band to taper off from southwest to northeast from around 12Z to 18Z Friday, resulting in gradually improving conditions during the day Friday, potentially reaching higher-end MVFR cigs or some VFR conditions before 00Z Saturday. Winds NE around 15 kts with gusts 20-25 kts, slowly decreasing to around 10 kts and shifting northward by around 12Z. && .ILX WATCHES/WARNINGS/ADVISORIES... Winter Weather Advisory until noon CST Thursday for ILZ027>031- 036>038-040>057-061>063-066>068. && $$ UPDATE...37 SYNOPSIS...Shimon SHORT TERM...Shimon LONG TERM...Shimon AVIATION...37
Area Forecast Discussion
National Weather Service Jackson MS
905 PM CST Wed Feb 5 2020 ...SEVERE WEATHER AND FLASH FLOODING REMAINS POSSIBLE THROUGH TONIGHT... .UPDATE... Updated for evening discussion. && .DISCUSSION... The bulk of the severe weather has moved east of the region, but the Tornado Watch does remain in effect for portions of eastern Mississippi through midnight. Storms continue to move north and east across this region ahead of a boundary that is generally located from Columbus to Brookhaven/Mccomb. West of this, a much more stable airmass is in place as temperatures have dropped into the 40s and lower 50s with north winds and dewpoints in the 40s and lower 50s. East of this, temperatures remain in the 60s with dewpoints in the upper 60s and lower 70s with a south wind. While the threat for severe weather is low in the watch area, there are enough parameters in place to keep the Tornado Watch in place for now. 1000 J/kg of MLCape, 40-60kts of shear, 300-500 m2/s2 of helicity all bring just enough risk for any storm to tap into this and quickly become severe. In addition, another area of ascent should come through later tonight as the upper trough nears and moves across the region. As a moist airmass is in place across the eastern part of the region, characterized by 1.7 inch PW values per SPC mesoanalysis, and the best moisture transport is in this region, there could also be a risk for flash flooding. Will keep mention of both of these threats through the rest of the evening into the early overnight periods. The upper trough should swing through the region tomorrow morning into the afternoon, clearing the rain and storms out. Going forecast was in decent shape and only minor adjustments were needed. /28/ Prior discussion below: Prior relevant morning discussion through this afternoon-evening: An active weather day is on tap today & will bring severe weather & flash flooding potential across the region. Severe weather, including tornadoes & some possibly strong, is likely & some flash flooding is possible. Rest of today: This morning the mid-level ridge axis is centered over the Caribbean Islands, leading to a an influx of sufficient low- level southerly return flow of warmth & moisture. Surface analysis this morning indicates a surface low & baroclinic zone across the ArklaMiss Delta, ArkLaTex to mid-South & will only deepen through the afternoon, while cooler conditions exist in the wake of the boundary in the northwest Delta. The synoptic pattern consists of a longwave amplification with the spoke of energy & jet streak energy through southern Plains & northern Mexico. This will continue to swing out along the base of the trough & eject northeast across the ArkLaMiss region. In conjunction with low- level deepening & surface cyclogenesis, strong reflection in the low-level wind field response will occur. Morning sounding analysis shows near the high end or climatological maximum in low- level observed destabilization (i.e. LCH surface based CAPE ~2500 J/kg & JAN surface/mix-layer CAPE around 800-1000 J/kg). This juxtaposition of steep mid-level lapse rates & strong mid-deep layer shear (i.e. 6.5 deg C/km lapse rates @ 700-500mb & 40-70kts+ effective bulk shear @ 0-3km & 0-6km, respectively) continue to support higher end severe weather potential. This low-level feed of high Theta E is leading convection development across areas in south-central to western Louisiana is serving as the impetus for convective development. This will continue to spread to the east-northeast across the region, with hi-res CAM guidance indicative of storms along the northern gradient of the significant tornado parameter gradient (STP). Local HRRR guidance is definitely showing the potential for some 1500-2000+ J/kg surface based CAPE along & southeast of that frontal & baroclinic zone & storms potentially riding along & southeast of that boundary. SPC mesoanalysis & forecast indicate nearly 2-3 STP along the Natchez Trace Corridor & especially from near Winnsboro up towards Columbus & to the south. This area also has pressure falls of 3-6mb as the surface low deepens. Hi-res CAM guidance continues to paint area in this vicinity with the highest potential for tornadoes, including some possibly strong. Exact storm mode may be somewhat of a question, per some other local CAM guidance, but there will be some potential for higher-end severe weather. This environment is anomalously destabilized & sheared. In conjunction with SPC, introduced an "Enhanced" severe risk area from near Vicksburg to the Jackson Metro & to the northeast to including near Grenada to the Golden Triangle. For now, all modes of severe weather are possible. Due to the strong clockwise curving hodographs, tornadoes are likely & some possibly strong. Damaging winds up to 70mph & large hail of up to golf ball size are also possible. These will be reflected in the HWO/graphics. In addition, the flash flooding potential remains across the area as there will be high moisture in conjunction with a deepening synoptic jet, upper diffluence along the low & also any convective potential. In conjunction with anomalous deep layer moisture & very wet antecedent conditions, especially from any heavy rain this morning, will keep potential around. Due to that, rainfall of 1-3 inches, with locally higher amounts, is possible across the region. Local hi-res CAM guidance & HREF suite continues to support this. No flash flood watch is needed for now as this mesoscale driven & some details are not as clear. Kept the HWO/graphics the same as before. Lastly, due to a strong developing pressure gradient some gusts upward of 25-30mph can`t be ruled out. Expect these winds to really ramp up into the afternoon as the surface low rapidly deepens & moves through the Delta. For now, winds don`t seem to ramp up enough for a wind advisory or mention in the HWO/graphics. /DC/ Tonight into Thursday: Tonight: As the longwave trough digs into southern Texas & northern Mexico, this will continue strong ascent, low-level surface pressure response (i.e. rapid deepening & cyclogenesis with surface low around or less than 1000mb) & feedback in the broad scale deep layer jet streak response. The upper level jet will rapidly deepen to nearly 100-150kts @ 300-500mb. Expect the severe potential to continue shift to the east-northeast, but severe potential could remain near or a few hours after midnight, especially for eastern-southeast Mississippi, especially on the eastern half of the Highway 84 corridor & Interstate 59-Highway 45 corridors. All modes of severe weather, including tornadoes, damaging winds & large hail, remain possible as well, but higher end severe weather becomes less likely than earlier in the afternoon-evening. In addition, this low pressure will continue to feed high Theta E air (i.e. ~330-335K 850mb Theta E) over mainly for areas along & southeast of the Natchez Trace. This will coincide with increasing ascent aloft for areas along & to the southeast. Due to that, some flash flooding potential will remain into the overnight hours. As of now it is difficult to pin down exactly areas of best rainfall totals, but most hi-res CAM guidance indicate potential for some localized 1-3 inches, with localized higher amounts, especially including rainfall totals through this evening. For now, will keep the graphic/HWO going as is & no flash flood watch for now. Thursday: Expect rapid drop in deep layer moisture (i.e. PWs falling from around an inch & a half to a half inch) in the wake of the cold front as strong northwest flow aloft & broad scale subsidence move. This will help rain chances to quickly clear out from the northwest through the afternoon. The highest rain chances will be near daybreak southeast of the Natchez Trace & continue to drift off to the southeast through the day. Expect a blustery afternoon as cold advection will keep low clouds & much cooler than normal high temperatures. Temperatures will struggle to reach the upper 40s in the Delta while highs will still only peak in the mid-upper 50s in the Pine Belt, Interstate 59 corridor & east Mississippi. /DC/ Thursday night through next Wednesday: Going into Thursday night, some precip will linger as moisture wraps around the backside of the exiting trough. With the cooler airmass moving in at that time, temperatures will drop into the 30s with some spots dropping to freezing in the north-northeastern part of the CWA. There is a chance that some of the lingering light rainfall after midnight could transition into light snow/flurries between midnight and sunrise, before the precip exits off to the east. No impacts are expected from the snowfall should it occur though. A lot of this will depend on how far the temperatures will fall in the northeastern CWA before the precip moves out. The cloud cover could inhibit the fall of temperatures so that the precip just remains as light rainfall. We`ll continue to watch this combination in future model runs. After the precip exits, Friday will be dry with seasonally normal temperatures as winds rotate out of the south again. Highs will be in the 50s areawide. By Saturday, a weak boundary swings around the base of an upper shortwave and triggers some light and scattered showers over our area. The global models vary on the strength of this boundary and the moisture availability so that some models do not show any rainfall over the area. At this time, it was decided to at least include some slight chances for rainfall. Again, we`ll see how the models react with future runs. The weak boundary will dissipate by Sunday and will leave Sunday warm and dry with highs increasing into the mid to upper 60s. Rain chances will be on the increase again though going into Sunday night and Monday as an upper shortwave trough dives down out of the Upper Plains. An accompanying frontal boundary will move through the CWA on Monday. With the good warm air advection ahead of the boundary and plenty of Gulf moisture available, some convective storms will be possible. This could lead to some heavy rainfall and possibly flash flooding. As the boundary lingers over the area through Tuesday, this certainly elevates the concern for heavy rain and flooding. With so much uncertainty in the models in the nearer term though , we`ll hold off on any mention of this in the HWO for now. Another rainy system looks to move through in the second half of next week, so it`s likely we`ll see some flooding at some point next week. Stay tuned for future updates concerning next week. /10/ && .AVIATION... 00Z TAF discussion: Widespread storms & gusty winds are ongoing across the region. The frontal boundary is currently situated across from areas near Natchez up near the Jackson metro & associated TAF sites (i.e. JAN/HKS) & just east of Grenada. This has shifted winds to more gusty to the north-northwest. Elsewhere winds are mainly from the south-southwest, which has led to more widespread storms. Flight categories have remained poor in most areas, especially in the wake of the front, near IFR ceilings, while to the east & south of the front are more closer to MVFR. Expect this front to pass through the overnight hours. Deteriorating flight categories will fall to LIFR ceilings, with drops to IFR visibilities/gusty winds in excess of 25-35mph, especially in any thunderstorm. Expect these light rain & flight categories to only slowly improve towards MVFR ceilings & improved visibilities after 06/17-20Z from northwest to southeast into Thursday. /DC/ && .PRELIMINARY POINT TEMPS/POPS... Jackson 47 50 33 56 / 89 61 22 0 Meridian 52 56 34 56 / 88 82 37 0 Vicksburg 47 49 33 57 / 77 39 4 0 Hattiesburg 54 58 34 57 / 97 82 9 0 Natchez 47 49 33 58 / 83 39 1 0 Greenville 42 44 32 52 / 69 32 13 0 Greenwood 44 46 33 52 / 85 43 36 0 && .JAN WATCHES/WARNINGS/ADVISORIES... MS...None. LA...None. AR...None. && $$ 28
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
933 PM CST Wed Feb 5 2020 .UPDATE... 933 PM...Evening update...overall current forecast in good shape. Main change this evening was to add freezing drizzle mention across the southern third or so of the cwa. There had been a few reports of freezing drizzle and currently freezing rain being reported at laf. When precip intensity increases...precip type will likely favor more snow but there will likely be periods where at least some light freezing drizzle will be possible. Snow has redeveloped and spread across much of the central cwa and short term guidance shows this continuing through the early morning hours. There appears to be an overall dissipating trend toward sunrise but confidence is fairly low for when accumulating snow...even if its light...actually ends and its possible there may be some light snow or snow showers persisting through the Thursday morning commute. Issued an SPS for much of the Chicago metro area...north of the advisory area for current snow. Only other minor change was to lower low temps tonight a few degrees based on current trends. Once the snow began this afternoon temps quickly lower into the mid 20s. Temps look to remain steady tonight with upper 20s/near 30 near the lake. cms && .SHORT TERM... 303 PM CST Through Thursday night... We continue to monitor the potential for accumulating snowfall this afternoon through Thursday across the area as well as possible lingering light lake effect/enhanced snow showers through the day on Thursday into Thursday night. The system responsible for dropping accumulating snow and sleet as far south as Texas continues to lift north and east toward the area. Current radar trends show radar returns have overspread our southern forecast area, mainly along and south of I-80. Earlier this afternoon a stout dry layer in the low levels aloft prevented the snow from reaching the ground, but this has since eroded with snow beginning to fall. Snowfall could pick up fairly quickly this afternoon and evening with moderate to briefly heavy snowfall possible under the heavier snow bands resulting in reduced visibilities. Sufficient upper level forcing with a semi-coupled jet stream pattern in place this afternoon situated under the right entrance of a departing jet and the left exit region of a strengthening jet streak. In addition, the banded nature of the snowfall is likely aided by 800mb frontogenesis that is translating north and east through the area this afternoon and evening. There can often be a sharper cutoff in precip totals to the north of these frontogenesis bands due to localized region of subsidence. This should delay the northward expansion of the snowfall across areas north of I-88, likely limiting snowfall totals there. Only made minor adjustments to the previous forecast. Adjusted totals down slightly especially for areas north and west of I-80. A swath of 3-4 inches of snow, with isolated 5 inch totals remains possible in the winter weather advisory area. Plenty of uncertainty remains in snowfall totals through the day on Thursday for areas along the lake. Models continue to suggest some minor lake enhancement as well as additional lake effect snow showers are possible late Wednesday night through the day on Thursday as the main system pushes off to the northeast and north northeast flow sets up across the lake. Warm lake temps and and convergence along the base of the lake suggest there is at least the potential for lake effect snow showers. However, as was mentioned in the previous discussion, upstream air is lacking moisture, which combined with the fairly low inversion heights indicated in model soundings, snowfall rates will likely be light. Petr && .LONG TERM... 223 PM CST Friday through Wednesday... Broad upper troughing over the Great Lakes region expected at the start of the long term period. Friday and Saturday look a lot like the stretch we had in January with cloudy skies, daytimes temps in the lower 30s, minimal diurnal range, and even a small threat of some light snow or flurries Friday night with a weak impulse rippling through the trough. Didn`t make any adjustments to the blended model low temp forecast for Friday or Saturday nights, however, seems like a good bet lows could be at least several degrees warmer. As confidence increases in cloud cover forecast, suspect later forecasts we will likely need to nudge these temps upward in later forecasts. Slightly more amplified shortwave trough is expected to move across the region Sunday into Sunday night. The 12z medium range guidance is in reasonably good agreement for this distance out and the consensus has a sfc low track near or north of our CWA. This would likely result in more liquid precip than frozen, though still a lot of time for things to change, so generally kept the rain/snow for p-type. In the wake of this wave, really not seeing any significant push of cold air. Another storm system looks to impact the region toward the middle of next week. Far too far out at this distance to discuss track, p-type, and other assorted details. - Izzi && .AVIATION... For the 00Z TAFs... Aviation forecast concerns: -Periods of light/moderate accumulating snow in IFR/possibly brief LIFR conditions into Thursday morning, with snow showers lingering through the period. -northeast winds gradually becoming more north-northeast Thursday, with some gusts into the 15-20 kt range. Surface low pressure over the lower Mississippi Valley will lift slowly northeast into the upper Ohio Valley through Thursday, in response to an upper level trough moving slowly east across the region. The local terminals will generally be on the northern periphery of the large scale precipitation shield with this storm system, with several smaller embedded disturbances modulating a few periods of more impactful snowfall, especially across the Chicago metro TAF sites. The first of these, associated with enhanced frontogenetic forcing in the 800-700 mb layer and upper divergence beneath coupled jet cores aloft, has spread across the southern part of the metro late this afternoon, and was already transitioning off to the east as of 530 pm. Various model guidance are in generally good agreement in developing another f-gen enhancement across the area this evening, beginning around 02Z, with slightly broader coverage depicted as well. While light snow may persist in between these bands, snowfall rates will decrease significantly with improvement in cig/vis conditions to MVFR or VFR for a time early this evening. IFR conditions and an uptick in snowfall rates will likely develop by mid-evening, with a return to IFR conditions and potentially some LIFR vis/cigs beneath the stronger banded elements. Have generally gone a little lower at MDW than ORD and at GYY than MDW, though confidence is lower in these details given the potentially broader banding depicted in simulated reflectivity guidance. This second band is expected to weaken around/after midnight, though again with some light snow persisting overnight. Some guidance, particularly NAM and 4km WRF runs, suggest another uptick however during the early morning hours. Confidence lowers with details further out in time, but have maintained an IFR vsby through about 14Z based on these guidance trends. Large scale forcing decreases by mid-morning Thursday, though with the main upper trough axis still approaching the potential for snow showers or periods of light snow will persist. Gradual cooling of the column will slowly increase lake effect snow shower potential during the afternoon and evening hours, as northeast or north-northeast low level flow continues. Model simulated reflectivity from NAM/4km WRF, RAP and HRRR all indicate an increase in snow shower coverage and intensity by early Thursday evening, which will likely affect the metro terminals, and have indicated a lower vsby in SHSN for ORD and MDW by 00Z Fri. Snow showers will likely persist beyond the end of the current TAF period, into early Friday morning, before winds turn more north- northwest and shift the lake effect focus into Indiana. KRFD will be on the far northwestern fringe of the better snow potential through the period. The best potential for light snow there looks to be late this evening into early Thursday morning, with mainly MVFR impacts to vsby and ceilings. Ratzer && .LOT WATCHES/WARNINGS/ADVISORIES... IL...Winter Weather Advisory...ILZ023-ILZ032-ILZ033-ILZ039 until 9 AM Thursday. IN...Winter Weather Advisory...INZ001-INZ002-INZ010-INZ011-INZ019 until 9 AM Thursday. LM...Small Craft Advisory...nearshore waters until 3 AM Friday. && $$ Visit us at weather.gov/chicago Follow us on Facebook, Twitter, and YouTube at: www.facebook.com/NWSChicago www.twitter.com/NWSChicago www.youtube.com/NWSChicago