FOUS30 KWBC 181600 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1200 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... ...Southwestern U.S.... The overall synoptic configuration remains largely unchanged from the previous period, with a persistent monsoon pattern locked in place -- keeping scattered to widespread convection in the forecast from West Texas into the Great Basin and southern Rockies. Given that PW anomalies remain notably high at 2-3 std dev above normal, and soil conditions moist from prior rainfall, confidence was sufficient to introduce a Slight Risk that extends from West Texas northeastward across southern New Mexico, central and northern Arizona, into southern Utah. An upper-level high centered over northern Utah and Colorado will support steady easterly flow across the southern tier, effectively tapping into and beginning to pull mid-level energy that has been centered over West Texas along this deeper moisture axis. The highest areal average precipitation amounts are expected to center along the higher terrain, where slow-moving storms are expected to develop with afternoon heating, before propagating off of the terrain along convective outflows. Given sensitive, moist soil conditions from successive days of widespread storms, much of this area remains vulnerable to rapid runoff. Complex terrain, slot canyons, and recent wildfire burn scars will further accentuate the threat for flash flooding across parts of the region. ...Mid Mississippi Valley east to the Carolinas and south to the Eastern Gulf Coast... A more expansive Marginal Risk has been drawn for this period, reflecting deep moisture atmospheric profiles and an increasing potential for localized heavy rainfall amounts producing isolated runoff concerns. This risk is centered along a cold front dropping into North Carolina and the Tennessee Valley, while is begins to slow over the lower Ohio and mid Mississippi valleys. Diurnal heating and prefrontal convergence will tap into this moisture pool, generating clusters of slow-moving efficient thunderstorms, capable of producing 1-2 inch totals, with isolated higher amounts. There remains a greater threat for more widespread heavier amounts along the eastern extent of this area, centered over southeastern Virginia and eastern North Carolina, where more defined upper forcing will help accentuate the threat for amounts over 3 inches. Additionally, the Marginal Risk has been extended southward down to the Gulf Coast. This accounts for the deep tropical moisture arriving ahead of the slow-moving disturbance lingering in the eastern Gulf, as well as favorable low-level convergence focusing along a slow-moving surface trough extending north from the Gulf. Highly efficient warm-rain processes along these features will maintain a threat for heavy, short-term duration totals and isolated runoff concerns. Pereira 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... The primary adjustment for this forecast cycle is the introduction of a Slight Risk across portions of New Mexico and Arizona. For parts of the region, the setup remains much the same, with anomalous moisture (2+ std dev above normal) supporting convective activity and rainfall efficiency. Mid-to-upper level energy/an inverted trough drifting east across the region will provide larger-scale ascent to help better organize monsoon convection. Furthermore, this feature will help steer and push the convective and heavy rainfall threat a bit farther west across portions of southeastern Arizona than in the previous days. Given the combination of persistent anomalous moisture, enhanced upper-level support, and sensitive soils, expect localized rainfall rates to easily raise runoff concerns across parts of the region. A surrounding broad Marginal Risk remains in place extending up into the Great Basin and central Rockies -- outlining the risk for less-organized storms, producing isolated runoff concerns in those area. ...Upper Great Lakes... A well-defined shortwave interacting with deepening moisture is expected to produce strong storms capable of producing heavy rainfall across the region. Guidance shows southwesterly to easterly inflow supporting PWs increasing to around 2 inches. This moisture along with strong ascent, afforded in part by favorable upper jet dynamics, will fuel the potential for heavy rainfall- producing storms. Training storms along the front may raise the threat for heavy amounts and flash flooding concerns. Models differ on the details, but there is some consensus for the heaviest amounts centering from southern Wisconsin to northern Illinois, where an upgrade to a Slight may be introduced in a future issuance, if the trends hold. ...Eastern North Carolina... 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. ...Florida Gulf Coast... It remains uncertain how the disturbance in the eastern Gulf will evolve and track. However, regardless of development, it is expected that heavy rainfall will remain a threat for parts of the Gulf Coast. Therefore, a Marginal Risk was maintained from the Tampa Bay region to the Panhandle. Pereira 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... ...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 $$