688 FOUS30 KWBC 181906 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 306 PM EDT Sat Jul 18 2026 Day 1 Valid 16Z Sat Jul 18 2026 - 12Z Sun Jul 19 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM PORTIONS OF WEST TEXAS THROUGH THE EASTERN GREAT BASIN, AS WELL AS PARTS OF THE NORTHEAST AND MID-ATLANTIC... ...Western Texas to the Four Corners/Great Basin... Monsoon ridge will become favorably aligned to draw impressive thermodynamics northward from Mexico, reaching from West Texas through much of the Desert Southwest and then northward into the Great Basin and Intermountain West. Multiple weak shortwaves embedded within the flow will continue to progress anti- cyclonically around this ridge, bringing locally enhanced ascent to the region, with a few of these impulses noted on morning WV imagery in TX and NM. While any of these impulses will locally enhance ascent leading to clusters of more organized convection today, the greatest risk is just the breadth of coverage of thunderstorms with potentially heavy rainfall today. PWs this morning are impressive across the region, measured via GPS and morning RAOBs vary from around 1.2 inches at SLC to as high as 2.1 inches at PHX, at or above the 90th percentile across a large area (and as high as +3 sigma), and that measured PW at PHX this morning is a daily record. At the same time instability is progged to climb to 1000-2000 J/kg, locally even higher, as far as 40N latitude in the Four Corners, which is impressive and is a typical discriminator of more widespread monsoonal heavy rain/flash flood days. THe CAMs, including the UA WRF, are aggressive with coverage of their simulated reflectivity across much of the area today, with hourly rainfall potentially reaching 1.5 to 2.5 inches. This could easily exceed the 1-hr FFG across the region, and in some areas the 3-hr FFG even has a high chance (40-60%) of being exceeded where terrain-tying or boundary-collisions can cause a temporal extension of the heavy rain rates. With storm motions generally expected to be weak at 5-10 kts and chaotic, an active day is expected, with scattered flash flooding expected, especially across vulnerable features like burn scars, slot canyons, and urban areas. The inherited SLGT risk was modified to account for recent guidance, and there was some consideration of an increased risk based on the impressive REFS exceedance probabilities which rise above 55% for FFG, but the more muted HREF was used more heavily, especially since the highest FFG exceedance is across the higher elevations of the Mogollon Rim and not down into the deserts (although an expansion SW into some lower terrain was made with this update). ...Midwest to the Mid Atlantic and Northeast... A cold front will drop steadily southeast from the Great Lakes, reaching the lower Ohio Valley and the Mid-Atlantic by Sunday morning. Ahead of this front, impressive moisture will pool from Kentucky eastward to Long Island and Cape Cod, where PWs are already 1.75 to 2.0 inches and will climb above 2.0 inches through this aftn. The overlap of this elevated PW with increasing SBCAPE (2000-3000 J/kg forecast) will create robust thermodynamics which will support widespread convection capable of producing intense hourly rainfall for which the HREF and REFS suggest has a 40-60% (10-15% chance) of exceeding 1 inch (2 inches). The excessive rainfall risk will steadily increase through the aftn and continue into this evening/tonight as multiple rounds of thunderstorms develop and track across the area. 0-6km bulk shear is progged to increase to 30-40 kts ahead of the cold front, and as weak impulses move west to east from the Ohio Valley into the Mid- Atlantic, this will allow for some organization into clusters tracking parallel to the front (and pre-frontal trough). Ahead of the front, the Corfidi vectors and mean flow will align from the west, leading to an increased backbuilding potential into the higher thermodynamics with cells then tracking across the same areas of the Mid-Atlantic states and into southern New England. This will occur ahead of the front which will produce a second round of heavy rainfall potential, albeit with more progressive NW to SE motion to the cells. The inherited SLGT risk has been adjusted to account for new guidance, and due to elevated probabilities from both the HREF and REFS (40-60% for 3"/24hrs, 10-15% for 5"/24hrs) atop sensitive areas due to urban development and recent rainfall producing compromised FFG. This included connecting the SLGT risks across PA where the ARW guidance is most aggressive and there is sufficient potential for some training (or meso-cyclones to enhance rainfall rates) to include a larger area. Considered an elevated internal SLGT risk for the Tri-State area as well, but enough uncertainty in position and timing of redevelopment after CAPE exhaustion this morning prevented that category increase. ...Florida Panhandle through Southwest Georgia... Deep moisture spreading north ahead of a slow-moving mid-to-upper level low will pool along a surface trough extending out of the eastern Gulf. This moisture (PWs increasing to above 2 inches) will fuel slow-moving storms as Corfidi vectors become aligned anti-parallel to weak 0-6km mean winds of just 5-10 kts. With rain rates possibly (20-40%) exceeding 2"/hr at times this could create at least isolated instances of excessive rainfall leading to flash flooding. ...Western Florida Peninsula... A convective MCV positioned west of the Florida Panhandle across the northeast Gulf will drift westward today within a pool of tropical PWs nearing 2.25 inches. As this feature drifts slowly westward, it has, at least this morning, temporarily stalled inflow along the Florida west coast between Naples and Fort Meyers. The high-res CAMs are not extremely aggressive with rainfall today, but have been under-forecasting the current activity (MRMS already 2-4" in some areas with hourly rain rates of 2-3"). If this band continues to stall, continued impressive rainfall rates are likely, and the recent RRFS, HRRR, and some of the experimental REFS members show an additional 2-4" of rain are possible. This has resulted in a narrow MRGL risk addition which aligns well with recently issued MPD #764. Weiss Day 2 Valid 12Z Sun Jul 19 2026 - 12Z Mon Jul 20 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS THE SOUTHWESTERN U.S. FROM WEST TEXAS TO SOUTHERN UTAH, AS WELL AS PORTIONS OF NORTH CAROLINA... ...Southwestern U.S.... The monsoon ridge will persist in a favorable alignment Sunday, keeping the overall synoptic pattern relatively unchanged from D1. This will create another day of scattered to widespread convection from the Desert Southwest, through West Texas, and northward into the Great Basin and Intermountain West. The primary different between the synoptic pattern Saturday vs Sunday may be just a subtle reorganization of the ridge center near the WY/CO border, which could allow for a subtly farther NW shift of thunderstorm activity than what occurs Saturday. Regardless, the environment will support another active day, and with PWs reaching +2 to +3 sigma (a plume of 0.75" north to 1.75" south) overlapping widespread MUCAPE of 250-1000 J/kg, this will support heavy rainfall rates within any of the convection on Sunday. HREF and REFS neighborhood probabilities indicate a 60-80% chance of hourly rainfall to 1", with locally 2"/hr rates also possible (10-20%). Although bulk shear is expected to remain modest, up to 20 kts, suggesting primarily pulse type convection, weak 0-6km mean winds and Corfidi vectors will allow for storm mergers and boundary interactions to cause chaotic and slow motions, with subtle terrain tying or boundary stalling storms developing at times. This will create locally 3-4" of rain, especially from the Mogollon Rim southeast into West Texas where higher MUCAPE is progged. However, anywhere these slow storms develop with the intense rain rates, instances of flash flooding will be possible, especially atop sensitive terrain features like slot canyons and dry washes, or over burn scars and urban areas. Once again, the probabilities indicate the heaviest rainfall will occur in the higher terrain, but light E/NE flow and outflows could help storms dive off the terrain into the lower deserts at times as well. ...Mid-Mississippi Valley through the Carolinas... After coordination with the eastern NC WFOs, a targeted SLGT risk has been added for eastern NC embedded within the broader MRGL extending from the Mid-Mississippi Valley through the Tennessee Valley and into the southern Mid-Atlanic states. This area will see scattered to widespread thunderstorm development along and ahead of a cold front which will slowly drop southward through the day. The downstream environment will be extremely supportive of heavy rain thanks to PWs of 2-2.25 inches overlapped with MUCAPE above 2000 J/kg. While much of the convection will be of the pulse variety, some increasing bulk shear reaching 25-35 kts, especially late across NC and the Mid-Mississippi Valley could help organize storms into clusters, which will then move parallel to the boundary to support prolonged periods of training. With rainfall rates of 2"/hr or more, this could cause locally more than 3" of rain, with a 20-40% chance of at least 5" across eastern NC where a shortwave will help spawn a wave of low pressure to improve ascent and organize thunderstorm activity. ...Eastern Gulf Coast... A tropical disturbance (AL91) for which NHC has a 40% of development over the next 7 days in the northeastern Gulf will spread at least moderate rainfall onshore the west coast of Florida through the Panhandle and potentially as far east as eastern Louisiana. While there is considerable uncertainty into the onshore extent of QPF on D2, the HREF and REFS both suggest a 20-40% chance of at least 3" of rainfall along the coast, driving the adjustments to the inherited MRGL risk. ...Upper Mississippi Valley... A shortwave embedded within squeezed mid-level flow will crest the Four-Corners ridge and then dive southeast Sunday evening/night into the Northern Plains and Upper Midwest. This shortwave will interact with a frontal system to enhance ascent, with a wave of low pressure potentially also developing. As the LLJ develops within the accompanying warm sector, it is progged to accelerate to 25-35 kts, drawing robust thermodynamics (PWs above 2" with MUCAPE over 3000 J/kg) northward and isentropically ascending the NW to SE oriented warm front. This will help spawn thunderstorms during the evening, with backbuilding/training possible from the eastern Dakotas into southern Minnesota. The HREF and REFS probabilities suggest a 15-25% chance for at least 3" of rainfall thanks to training 1-2"/hr rain rates, so a MRGL risk was added for at least localized flash flood potential. Weiss Day 3 Valid 12Z Mon Jul 20 2026 - 12Z Tue Jul 21 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF ARIZONA AND NEW MEXICO... ...Southwest... Changes from inherited were primarily cosmetic to account for new guidance, but another day of active (although with somewhat less coverage than the weekend) monsoonal convection is expected. This is due to a generally unchanged synoptic pattern with anomalous moisture supporting convective activity and rainfall efficiency. South of the ridge, as in the previous few days, shortwaves rotating westward will help steer and enhance convective coverage, and where this overlaps with the greatest instability/PW across NM/AZ should receive the greatest coverage of thunderstorms with flash flood potential. There is some concern that instability may struggle to return Monday due to widespread convective coverage and residual cloud cover Sunday night, which may necessitate additional adjustments to the risk area, but at this time it still appears there will be another day of scattered to widespread convection with at least scattered flash flood instances. This will be most likely across vulnerable features such as dry washes, burn scars, and slot canyons. ...Great Lakes... A well-defined shortwave interacting with deepening moisture is expected to produce strong storms capable of producing heavy rainfall across the region. Locally backed and accelerated low- level flow will help surge PWs to 2 inches or more, especially within a progressive warm sector progged to race across the region during D3. This moisture, along with strong ascent driven both by low-level isentropic ascent and favorable upper jet dynamics, will fuel the potential for heavy rainfall-producing storms. Increasing bulk shear to 30-50 kts will help organize convection into clusters or supercells, raising the potential for intense rain rates of 1-2"/hr, through which any short-term training could enhance the flood risk. At this time storms may move rapidly enough to preclude a higher risk, but will continue to monitor for a possible targeted upgrade to a SLGT risk with later issuances. ...North Carolina/Southern Virginia... A Marginal Risk was maintained where a front and coinciding pool of deep moisture (PWs at or above 2 inches) are expected to linger, extending the threat for heavy rainfall and possible flash flooding into a second day. There is some potential a SLGT risk may be needed here as well, but that risk will be somewhat dependent on what occurs D2, so after coordination with the WFOs, opted to maintain the MRGL for now and reassess as the event gets closer. ...Florida Gulf Coast... Considerable uncertainty remains regarding AL91 in the northern Gulf. There will continue to be at least a low-end risk for heavy rainfall around this system as tropical moisture streams northward to the FL Panhandle and surrounding areas. At this time the MRGL risk was maintained and only cosmetically adjusted, but future refinements are expected as this system becomes more organized. Weiss Day 4 and Day 5 Valid 12Z Tue Jul 21 2026 - 12Z Thu Jul 23 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHWEST AS WELL AS THE CENTRAL APPALACHIANS ON TUESDAY... ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR EASTERN NORTH CAROLINA AND SOUTHEASTERN VIRGINIA ON WEDNESDAY... 2030Z Discussion... Few changes needed to either the excessive risk in the eastern US or the western states. The 12Z model QPF and the ensemble guidance still aligned fairly closely with previous runs...so changes to the Day 4/5 ERO were minor adjustments rather than a shift in the overall forecast reasoning, Bann Previous Discussion... ...Southwest... The Southwest Monsoon will remain in full force across Arizona and into Utah on Tuesday. Moisture anomalies especially to the north across Utah could approach 5 sigma above normal for this time of year, while into Arizona, PWATs to 2 inches will be a solid 3 sigma above normal. With daytime heating, renewed rounds of thunderstorms are expected to redevelop for the latest in the multi-afternoon pattern. Soil moisture levels are already saturated over much of Arizona and into southern Utah. Expect with the thermal low peaking in intensity on Tuesday, that flash flooding concerns will become more widespread as rains impact beyond the 4 Corners into Nevada, Idaho, and Wyoming. A Slight Risk upgrade was introduced with this update, and it's possible localized areas may see further upgrades as Tuesday approaches. By Wednesday, ridging over the Southwest will push the greater threats for thunderstorms further north into the Intermountain West, with a low developing over the Plains allowing for some of the moisture to overspread into portions of the central Plains. The signal for heavy rain lessens on Wednesday, so the threat drops to Marginal for now, though a Slight may be needed into the Intermountain West with continued consistency in the guidance. ...Central Appalachians... A Slight Risk upgrade was also introduced for portions of the Central Appalachians, particularly across West Virginia and into southwest Pennsylvania. Abundant moisture across the Mid-Atlantic will interact with a strong cold front pushing southwest across Ohio during the afternoon. This will allow for numerous thunderstorms to develop Tuesday afternoon. Upslope flow into the flash flood prone mountains of West Virginia and Pennsylvania will support additional thunderstorm development as compared with areas further west where the front will clear the moisture, and further east where there will be less forcing. PWATs approaching 2 inches will also support storms capable of multiple inch per hour rainfall rates, especially as they run into the mountains. To the east across the Mid-Atlantic, moisture levels further increase, with some areas perhaps reaching 2.25 inch PWATs Tuesday afternoon. The forcing will be a bit more uncertain, but any storms that form will be capable of very heavy rains. The I-95 corridor from DC to NYC will have to be closely monitored as these heavily populated and flood prone areas may have enough widespread activity to need further upgrades. ...Tidewater of North Carolina and Virginia... On Day 5/Wednesday, the same cold front that swings through the Midwest, Northeast, and Mid-Atlantic on Tuesday slows down its forward speed, perhaps even stalling to the west of the Tidewater of North Carolina and Virginia. Plentiful tropical moisture continues riding northeastward along that front, providing ample opportunity for numerous training showers and thunderstorms to develop over the region, likely moving out over the Atlantic off Ocean City, Maryland and points north. Thus, the focus will be in the highly urbanized Hampton Roads area, extending into much of the coastal plain of North Carolina. Given the pretty impressive signal for heavy rain, have started out the Day 5 ERO with a Slight Risk in this region. There remains some uncertainty as to whether any tropical development over the northeastern Gulf contributes to the heavy rainfall expected in this region on Wednesday. Given the signals for heavy rain expected Sunday/Monday over eastern North Carolina, expect the soils by Wednesday to be significantly more saturated than they are now, contributing to the heightened flash flooding risk. Wegman Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt $$