Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Tue Aug 01 2023 07:58:00 FOUS30 KWBC 010810 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 410 AM EDT Tue Aug 01 2023 Day 1 Valid 12Z Tue Aug 01 2023 - 12Z Wed Aug 02 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS MISSOURI AS WELL AS THE CENTRAL ROCKIES AND PLAINS... ....Southwest to the central High Plains... Another round of afternoon to evening convection expected today with the monsoonal moisture axis primarily confined to portions of eastern Nevada through much of Utah and into Wyoming/Colorado. Probabilities of seeing 0.5-1" hourly totals ramp up considerably by afternoon, peaking upwards of 30-50 percent across portions of NV/UT and above 60-70 percent for the CO foothills eastward through the central High Plains by late tonight. Some of these areas have received heavy rainfall Monday and early this morning, so soils will become increasingly saturated, especially across portions of CO. So the combination of intense rain rates (1-2"/hr), isolated rainfall totals of 3-4", and wetter antecedent conditions may lead to another round of scattered flash flooding. ....Northern Plains... Localized heavy rainfall will be possible during the period as an axis of higher moisture surges ahead of an approaching front combined with favorable forcing and enough instability to produce localized intense thunderstorms that may lead to a few instances of training thunderstorms that could produce flash flooding. ....Mid Missouri to the lower Ohio Valley... Very active, unsettled, and potentially significant heavy rainfall event beginning to set up across portions of Missouri the next couple of days. Upper ridge axis remains anchored over Texas early this morning with the ring of fire convection developing along its periphery. The northwest flow aloft positioned over the Mid Missouri to Mid Mississippi Valleys will interact with the strengthening low level jet early this morning to produce a narrow/localized swath of heavy rainfall that is just beginning to develop per recent radar and satellite imagery. This activity is likely to train/repeat as it slowly drifts southeast across central Missouri. The 00Z HREF suggests localized higher totals exceeding 3-5" will be possible through late morning before the low level jet wanes. This generally accounts for the western half of the Slight Risk area, which is on the higher end of the Slight Risk probabilities. After a break/lull expected mid day to early evening, another round of convection will develop across portions of central/eastern Missouri after midnight tonight through Wednesday mid morning. The setup continues to look very conducive for heavy rainfall with anomalous moisture in place, sufficient elevated instability, and a strengthening low level jet positioned favorably into the region. The 00Z HREF and CAMs show potential for another localized/significant heavy rainfall swath, which is likely to be displaced just enough eastward of this morning's heavy rainfall, limiting the overlap and this threat accounts for the eastern area of the Slight Risk outlook. That threat will continue into the Day 2 period with additional heavy rainfall going beyond 12Z Wednesday. Taylor Day 2 Valid 12Z Tue Aug 01 2023 - 12Z Wed Aug 02 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE MID MISSISSIPPI VALLEY... ....Mid-Mississippi and Lower Ohio Valleys... Convection will be ongoing at the start of the period across portions of Missouri, associated with the nighttime low level jet interacting with a frontal boundary draped in the region and shortwave energy moving through the northwest flow aloft. Favorable moisture axis and sufficient elevated instability will continue the threat of locally significant heavy rainfall and flash flooding early on in the period Wednesday morning where the latest deterministic and ensemble guidance supports potential for several inches. This should fall slightly displaced from the heavy rainfall on Tuesday morning, but also could be more significant given the more favorable environmental ingredients. In coordination with WFO St. Louis, a Moderate Risk was introduced to account for the potential for locally but significant flash flooding early Wednesday. Another round of heavy rainfall is also then likely to develop again Wednesday night into Thursday morning, though with a warm front beginning to advance more eastward, this axis of heavy rainfall looks to be displaced eastward yet again but could fall near or around the St. Louis metro area and into southern Illinois and with similar environmental ingredients (high PWs, warm cloud depths, strengthening low level jet), swaths of heavy rainfall will be likely at the end of the period into Thursday morning. ....Great Basin to North/Central Rockies and High Plains... Widespread showers and thunderstorms are expected Wednesday into Wednesday night aided by shortwave energy and right entrance dynamics with a strengthening jet streak over the northern Rockies. Moisture will be very anomalous, upwards of 1-1.25" of precipitable water and nearly 3 standard deviations above normal, based on the latest model guidance and with the daytime heating and terrain influences, localized intense rain rates exceeding 0.5"/hr will be possible. The 00Z guidance continues to highlight portions of eastern Idaho, western Wyoming, and northern Utah with the greater probabilities of seeing the higher rainfall totals as well as portions of Colorado and southeast Wyoming where Slight Risks were maintained from the previous forecast cycle. Taylor Day 3 Valid 12Z Wed Aug 02 2023 - 12Z Thu Aug 03 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE TENNESSEE VALLEY AND NORTHERN HIGH PLAINS... ....Tennessee Valley... A pair of robust shortwave troughs are expected to move through portions of the Mid Mississippi to Tennessee Valleys Wednesday night (early Thursday) and then again Thursday night (early Friday), bringing a couple rounds of heavy rainfall to the area. Convection is expected to ramp up late in the evening and overnight hours associated with the strengthening low level jet where the latest model guidance shows upwards of 30-40 kts at 850 mb impinging on a boundary draped across the region. With the mean flow oriented nearly parallel to the expected storm motions, some repeating rounds and backbuilding will be possible and given the anomalous moisture expected (PWs 1.75-2"), the highly efficient rain producing thunderstorms combined with potential longer duration could bring localized swaths of higher end rainfall totals. There still remains large model spread, particularly spatially with some north/south uncertainties but the consensus and ensemble data point toward portions of southern Kentucky through middle and eastern Tennessee having the grater threat for both rounds to overlap for the period, so a Slight Risk was introduced this forecast cycle. ....Great Basin to Northern High Plains... Shortwave energy lifting through the region combined with favorable right entrance jet dynamics will support a broad area of forcing for ascent from portions of the Great Basin to the Northern High Plains. Moisture anomalies will remain above normal, nearly 2-2.5 standard deviations above normal, particularly across portions of northeast WY, southeast MT and western SD. Convection is expected to develop initially over portions of the Great Basin before moving eastward through WY/MT and pushing into the Plains later in the period where it'll interact with the greater instability/moisture. Localized rain rates 0.5-1"/hr will be possible across ID while further east rain rates up to 1.5"/hr will be possible in the Slight Risk area, where the 00Z guidance suggests localized rainfall totals over 2" will be possible. The inherited Slight Risk continues to look reasonable with only minor adjustments made. $$ --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .