Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Aug 31 2023 03:41:37 FOUS30 KWBC 310341 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1140 PM EDT Wed Aug 30 2023 Day 1 Valid 0331Z Thu Aug 31 2023 - 12Z Thu Aug 31 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER THE EASTERN CAROLINAS... ....0331Z Special Excessive Rainfall Discussion... Modified the on-going Day 1 Excessive Rainfall Outlook based on trends seen in the radar imagery...largely to remove the Moderate Risk area where rainfall had diminished behind the Idalia. With moist confluent flow in the area...felt it was still warranted to keep a Marginal in some areas. Refer to Mesoscale Precipitation Discussion 1013 for additional details. Bann ....Previous Excessive Rainfall Discussion... ....Carolinas... Convergence of tropically soused southerly flow ahead and right of Idalia with northeasterly flow ahead and left (and behind a slow moving cold front that has pushed from VA into NC this evening) will continue to allow a broken axis of extremely heavy rain to fall in advance of the center of Idalia. This axis will continue shifting from east-central SC through south-central and eastern NC overnight. Additional rainfall of 4-6" is likely through 12Z where the Moderate Risk is in place. Intermittent downpours can be expected in the saturated air between the convergence line and the coast. Considerable flash flooding is expected to continue through this corridor. ....Florida into Georgia... Wrap around flow behind Idalia continues to bring dry air advection to GA/AL with an axis of lingering over northern GA should work south along the GA/AL line to the FL Panhandle overnight where the Marginal Risk is maintained. This northerly flow veers northwesterly over the Gulf off the FL Panhandle which converges with southwesterly flow from the tropical high over the Caribbean over the central FL Peninsula including and south of the Tampa Bay metro. Ample moisture and instability should keep a corridor of locally heavy activity into the overnight where the Slight Risk is maintained. ....New England... The initial upper trough axis has moved through Maine with the second axis coming from Quebec overnight to bring light to moderate showers. The Marginal Risk has been removed. ....Northern Rockies... The upper low is now over eastern MT and will continue drifting ENE overnight. Associated progressive activity will remain so overnight. Remaining stratiform rain over the Glacier Park region/Lewis Range is not expected to be excessive. The Marginal Risk has been removed. Jackson=20 Day 2 Valid 12Z Thu Aug 31 2023 - 12Z Fri Sep 01 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER A PORTION OF EASTERN NORTH CAROLINA BEFORE IDALIA HEADS OUT TO SEA... 2030Z Update... Given the progressive nature of Idalia and with support from the HREF, trimmed away some of the western extent of the previous outlook areas over eastern NC. Farther south, expanded the Marginal Risk to include northern Florida. The 12Z guidance, including the HREF shows persistent westerly flow supporting deep moisture (PWs at or above 2 inches) and the potential for additional convection producing locally heavy amounts. The HREF indicates locally heavy amounts of 2-3 inches are likely, which may cause runoff concerns, especially for areas that have already been impacted with heavy rainfall amounts. Pereira Previous Discussion... ....Eastern North Carolina... Introduced a Moderate Risk of excessive rainfall as Idalia makes its turn towards the east over North Carolina and makes its way out to sea. WPC QPF was still had localized rainfall maxima in of excess of 5 inches over the eastern part of North Carolina during the Day 3 period. Because the rainfall pattern is expected to be asymmetric by the start of the Day 2 period...the decision was made to introduce a Moderate risk area where the WPC combined rainfall from Day 1 and Day 2 was approaching 7 or 8 inches of rain by the time the rain tapers off. The 00Z HREF 40 km neighborhood probabilities still exceed 20 percent for 5 inch amounts in the first 6 hours of Day 2. No other major changes were necessary given the proposed track from the National Hurricane Center with bounded regions supported by ensemble and deterministic output. ....Western U.S.... Enhancement within the monsoonal pattern arises by Thursday as a deepening longwave trough over the west coast in tandem with a building upper ridge center over AZ/NM creates a funnel for increased low to mid-level moisture over the Southwest U.S, mainly AZ up through UT. Modest instability between 1000-1500 J/kg is forecast up through the aforementioned area with several deterministic outputs placing scattered to widespread convective coverage from the southern border up through the Mogollon Rim into UT. Locally heavy rainfall within any convective development will offer a threat for flash flooding, especially near any terrain or urbanized corridors.=20 Farther north...the amplification of that longwave trough along the west coast should result in a growing area of rainfall mainly over Oregon. Amounts tended to be on the marginal side for an outlook area but the concern about flooding or run off from burn scars warranted keeping the previously issued Marginal risk area in place with only minor adjustments.=20 Bann Day 3 Valid 12Z Fri Sep 01 2023 - 12Z Sat Sep 02 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ALONG THE GULF COAST AND OVER THE SOUTHWEST UNITED STATES... 2030Z Update... Made only minor adjustments based on the 12Z guidance. Pereira Previous Discussion... ....Gulf Coast... The models draw a mid- and upper-level impulse northward from the Gulf...accompanied by a surge of moisture and areas of rain.=20 Given the magnitude of the precipitable water and moisture transport...locally heavy rainfall is expected. The models offer a range of solution in terms of placement and amounts...with the NAM being more aggressive with highest rainfall amounts well east of other guidance and the GFS was a northern outlier. For now...tended to favor a consensus approach in placement.=20 Introduced a Slight risk where there could be some sensitivity about run off where model QPF overlapped rainfall from Idalia.=20 Confidence is below average given the spread and how much overlap there actually will be.=20 ....Southwest United States... Introduced a Slight Risk area along and near the higher terrain from Arizona to Utah....where moisture gets channeled northward efficiently between an anomalously deep trough along the west coast of North America and a pronounced ridge axis extending from New Mexico into parts of Colorado. Precipitable water values over southwest Arizona early in be period in excess of 1.5 inches get drawn towards the higher terrain over central Arizona by 850 mb flow of 15 to 20 kts by late afternoon...with values in excess of three-quarters of an inch along and near the terrain into Utah by late afternoon. The moisture and resulting instability sets the stage for some of the storms to be locally heavy rainfall producers...especially from southwest Utah or northwest Arizona given the approach of the surface front and mid-level height falls on Friday. WPC QPF was comparable with guidance showing areal averaged rainfall amounts around 1.5 inches. Of particular concern would be for flash flooding in sensitive slot canyons and dry washes. The placement of the WPC Slight Risk covers the corridor of highest model QPF and along the axis of the particularly strong moisture vapor transport shown by the NAEFS and EC ensembles. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7r-dwUheosE4o2zA_fAcswy53uWUeR0AD8WPBEbg1Moq= VownfUmOyotTJJN7_I9Dnnyt33G_vzZhhWN85SSTVHo4STM$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7r-dwUheosE4o2zA_fAcswy53uWUeR0AD8WPBEbg1Moq= VownfUmOyotTJJN7_I9Dnnyt33G_vzZhhWN85SSThoWgzH0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7r-dwUheosE4o2zA_fAcswy53uWUeR0AD8WPBEbg1Moq= VownfUmOyotTJJN7_I9Dnnyt33G_vzZhhWN85SST8BqvDPY$=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. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .