Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Mar 03 2023 00:43:34 FOUS30 KWBC 030043 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 743 PM EST Thu Mar 02 2023 Day 1 Valid 01Z Fri Mar 03 2023 - 12Z Fri Mar 03 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FROM NORTH TEXAS AND EASTERN OKLAHOMA INTO ARKANSAS AND THE MID-MISSISSIPPI VALLEY... There is an increasing threat for excessive rainfall this evening from portions of northern Texas and eastern/southeastern Oklahoma eastward across Arkansas into portions of the Mississippi Valley this evening and through the overnight hours. Early evening satellite and radar imagery showed convection increasing in strength and areal coverage from Texas into Oklahoma as an anomalously deep upper low emerges from the southern Rockies and the adjacent Plains. Between the increasingly difluent flow aloft with an active dry-line/boundary extending southwestward from near the Red River into west Texas being one focus for convection capable of producing rainfall rates in excess of 1.5 inches per hour that will shift eastward during the evening. MRMS already showed pockets if 3-hourly rainfall in excess of 3 inches and several places where 1-hour amounts were approaching 2 inches per hour. A second area of concern is where a low level jet nosing northward from the Gulf into northwest Louisiana and southeast Oklahoma should result in storms becoming increasingly efficient rainfall producers that shifts east-northeastward across Arkansas and into portions of the Mid-Mississippi Valley later this evening and during the overnight. Of particular concern is the fact that parts of Arkansas were drenched on Wednesday evening into early Thursday and present indications are that some of the activity tonight will track over parts of the same area. 00Z soundings already showed precipitable water values approaching 1.7 inches along an axis of LCH to SHV. The coverage of heavy to excessive rainfall is expected to increase as cyclogenesis accelerates later tonight in response to the approach of the upper system. Except for trimming a small portion if the area out of the Excessive Rainfall Outlook where convection has already moved away...only minor adjustments were made to the outlook areas elsewhere. Refer to MPDs 083 and 084 for latest mesoscale details. Bann Day 2 Valid 12Z Fri Mar 03 2023 - 12Z Sat Mar 04 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL THROUGHOUT MUCH OF THE OHIO VALLEY... A potent, negatively tilted, mid to upper level shortwave is likely to translate northeastward from the MO/AR border into the OH Valley through Friday evening, followed by weakening into the Northeast through Saturday morning. At the surface, a deep surface low will track into IN/OH Friday afternoon with an occluded cyclone sweeping eastward to the south of the surface low. A triple point low is forecast to develop over the Mid-Atlantic region with the main warm conveyor shifting to the East Coast for 00Z with rapid low level moisture transport from the northern Mid-Atlantic into Long Island and nearby locations. Heavy rain is expected to be ongoing at the start of the period (12Z Friday) across the Lower Ohio Valley where training and repeating of rain is expected, with 1-2 in/hr rainfall rates just ahead of the surface low track along an eastward extending baroclinic zone, at the nose of a 50-70 kt 850 mb jet. While there is some uncertainty with how quickly heavy rain from the Day 1 period will shift east for the beginning of Day 2, strong upper level diffluence and coupled divergence will be present over the OH Valley within the left exit region of a 130-150 kt jet near 300 mb and the right entrance region of an anticyclonically curved jet stretching west to east across the Upper Midwest into the Northeast. Instability is an unknown, with only ~500 J/kg MUCAPE forecast over southern IL/IN/OH during the day, not considering the more aggressive 12Z NAM which had a less agreeable surface to mid-level low low track. Beyond ~00Z, instability is expected to only be a couple hundred J/kg as precipitation moves toward the eastern U.S. Portions of the risk areas have received well above average precipitation over the past 2 weeks which may play a role in runoff given elevated soil moisture content. The Slight Risk was drawn for the highest probabilities of 2 to 4 inches over the 24 hour period (12Z HREF and global guidance), centered just north of the OH River. Northern portions of the outlook area were trimmed southward a bit to account for likely frozen p-types, but there is enough uncertainty with the track of snow/ice to keep a Marginal Risk from portions of central IL into northern IN and northwestern OH. Farther east and beyond 00Z, the threat for excessive rainfall appears low but enough to support possible exceedance of the low 3 and 6-hr FFG in place across parts of the northern Mid-Atlantic region where rainfall rates locally over 1 in/hr and 1 to 2 inches in 3 hours will be possible. Across southern areas (TN into northern MS/AL/GA), the Marginal Risk was left in place given heavy rainfall which fell Wednesday night into this afternoon. While a progressive line of showers/thunderstorms along/ahead of the cold front is expected, very localized flooding could occur if a stronger cell or two is able to materialize in advance of the main line and briefly train over a sensitive location, posing a risk for very isolated flooding. Otto Day 3 Valid 12Z Sat Mar 04 2023 - 12Z Sun Mar 05 2023 ....THERE IS A MARGINAL RISK FOR COASTAL AREAS OF SOUTHERN OREGON AND NORTHERN CALIFORNIA... The busy pattern in the Northwest looks to continue as another deep upper low develops off the Pacific Northwest coast late Friday into Saturday. A shortwave on the south side of the mid-level closed low is likely to impact the coast of OR into northern CA at the start of the Day 3 period (Saturday morning) with a round of locally heavy rain. A second round of heavy rain is anticipated in the 00-12Z Sunday window, tied to another mid-level shortwave rounding the base of the closed low. Each shortwave is forecast to be accompanied by a surge in 850 mb winds of 40 to 50 kt, with wind direction ranging from S to W over the period, impacting the lower elevations of the Coastal Ranges. While precipitable water values do not look to exceed 0.5" along the coast and snow levels will be near 2000 ft ASL, lowering into Sunday morning, a localized threat for flooding will exist given the potential for locally heavy rain from the two rounds of precipitation. Steep lapse rates will be present from 850 mb to 500 mb (or above 500 mb given a low tropopause height) which should promote a showery regime with embedded heavier cores. 24 hour totals of 1-2 inches are expected along the coast but with localized totals near or just above 3 inches possible. Recent rain/snow has helped soil moisture to be above average for the region and WFO EKA noted minor flooding on some area roadways, so the additional rainfall from Saturday into Sunday could renew localized flooding concerns. As the event draws nearer, future forecast guidance may support adjustments or complete removal of the Marginal Risk area. Otto Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!75uFvLSX-tjSBmK4GhoWKR27Loc9p6XZtwPnJpO7mmtr= _Jx5t6PZJpwL3u_iPuUo1ydlnQbOVCzGOPK29RGLJpY3ZBY$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!75uFvLSX-tjSBmK4GhoWKR27Loc9p6XZtwPnJpO7mmtr= _Jx5t6PZJpwL3u_iPuUo1ydlnQbOVCzGOPK29RGLVifzLc8$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!75uFvLSX-tjSBmK4GhoWKR27Loc9p6XZtwPnJpO7mmtr= _Jx5t6PZJpwL3u_iPuUo1ydlnQbOVCzGOPK29RGLJGxCLM4$=20 $$ =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. 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