Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Sep 02 2023 19:55:33 FOUS30 KWBC 021955 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 354 PM EDT Sat Sep 02 2023 Day 1 Valid 1954Z Sat Sep 02 2023 - 12Z Sun Sep 03 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHWEST... ....20Z Special Update... The Slight Risk area along the Gulf Coast was removed with this update in coordination with MOB/Mobile, AL and TAE/Tallahassee, FL forecast offices. Any weak convection this afternoon has remained well offshore, with any convection over land expected to remain west of the previous Slight Risk area into MS/LA. Wegman ....Previous Discussion... ....Arizona to southeast Nevada and northward into Utah... Another round of locally strong convection is expected this afternoon/evening across portions of the Southwest into the Intermountain West in response to the mid-level closed low over California helping to channel anomalously high moisture northward through the region. Compared to yesterday, the moisture axis isn't as deep though instability is expected to top out around 1000-1500 J/kg MLCAPE especially across western AZ, southwest UT, and portions of southern Nevada. This should support intense rain rates greater than 0.5-0.75"/hr at times with the strongest cores with a few localized/isolated 1"+ hourly totals possible. Based on the 00Z guidance, localized 1-2" totals will be possible and combined with the heavy rainfall footprint from Friday, some greater sensitivity is likely to be present so the Slight Risk still looks reasonable and covers where 2-day totals may exceed 3-4" in spots over some more vulnerable locations. ....West Coast... Closed mid-level low is forecast to remain nearly stationary over the course of the period, generally across northern California and southern Oregon. The lower heights contributing to steeper lapse rates combined with at least some higher than normal moisture and potential for marginal instability should support some stronger showers and perhaps thunderstorms capable of producing locally intense rain rates greater than 0.5"/hr. This may lead to a few localized flooding concerns particularly for the terrain areas of northern California and around recent burn scars. ....Gulf Coast... The axis of deep moisture, characterized by precipitable water values in excess of 2-2.25", will remain along the Gulf coast, aided in part by onshore flow provided by mid level low positioned over portions of eastern Texas and western Louisiana. The strong onshore flow will overrun a nearly stationary boundary draped west to east onshore and with a steady surge of low level moisture transport, expect steady/repeating showers and thunderstorms through much of the day across the central Gulf Coast. The high PWs and warm cloud depths will support efficient rain producing thunderstorms with hourly totals likely to be 1-2" at times. The best instability will remain generally confined to the immediate coast where model guidance and 00Z HREF neighborhood probabilities continue to suggest 2-3" totals (isolated 3-5+") will be common. Given the favorable setup for training/repeating rounds and very efficient/intense rain rates, a Slight Risk was introduced along the immediate coastal areas lining up with the greatest HREF probabilities for 3"+. The intense rainfall and rain rates may lead to isolated/localized instances of flash flooding for mainly urban and low-lying locations. Taylor Day 2 Valid 12Z Sun Sep 03 2023 - 12Z Mon Sep 04 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE INTERMOUNTAIN WEST... ....Intermountain West... A closed mid-level low is expected to move from coastal region of northern California through the Intermountain West by early Monday morning. Ahead of that feature, large/broad scale forcing for ascent will be found aided in part by the left exit region of a jet streak rounding the base of the trough across NV. Meanwhile, precipitable water values are expected to be nearly 3 standard deviations above the climatological mean across northern Utah, southern Idaho, and northern Nevada with values approaching 1". The combination of the deep forcing and anomalous moisture along with some instability (MLCAPE of 500-800 J/kg), widespread showers and thunderstorms with embedded stronger cores are likely during the afternoon/evening hours. 00Z HREF neighborhood probabilities for 0.5" hourly totals peak at 50-70% across NV/ID/UT with some signal (10-15%) for 1" hourly totals. The Slight Risk area was minimally changed from the previous update and aligns well with the best overlap of forcing, moisture, and potential instability where isolated/scattered flooding impacts will be most likely. Taylor Day 3 Valid 12Z Mon Sep 04 2023 - 12Z Tue Sep 05 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE NORTHERN ROCKIES... ....Northern Rockies and Northern Plains... The approach of the mid-level shortwave energy through the Intermountain West and Northern Rockies will provide the large scale forcing for ascent across the region Monday into Monday night. Meanwhile, anomalous precipitable water values lingering from the surge of deep monsoonal moisture will be found across much of northern Wyoming through southern/southeast Montana, characterized by PWs near 1" to locally 1.25", which is nearly 2.5-3 standard deviations above the climatological mean. During the peak heating, 00Z guidance does show a signal for an axis of 500+ J/kg of MLCAPE to develop to help promote stronger cores and more intense rain rates. Overall, a widespread moderate rainfall event is likely with embedded stronger cores across far northern Wyoming and southern Montana where localized 1-2" totals for the 24-hr period will be possible and this may lead to localized/isolated instances of flash flooding. Further east, low pressure ejecting out into the Plains Monday night into early Tuesday will promote thunderstorm development across the northern Plains (northern South Dakota through central North Dakota). A few stronger thunderstorms repeating over the same area will be possible and may lead to isolated flooding impacts. Taylor Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9ibixm4_SFNDkDncfNfGw9K31b2PivhSzvjBDzAenTFN= DCOIGRITZvBOaxdBkof_-AUgwBErTvH73bW06Sl6cDMAzxc$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9ibixm4_SFNDkDncfNfGw9K31b2PivhSzvjBDzAenTFN= DCOIGRITZvBOaxdBkof_-AUgwBErTvH73bW06Sl6eF4f-JY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9ibixm4_SFNDkDncfNfGw9K31b2PivhSzvjBDzAenTFN= DCOIGRITZvBOaxdBkof_-AUgwBErTvH73bW06Sl6UAUJ_dk$=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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