29 SET 2026 · Weather Preparation Article
Monday, September 28, 2026 — 7:53 PM CDT
Prepared for: Chicago 60645 and St. Louis, MO
All data sourced from the National Weather Service and NOAA centers
I. Executive Summary
A multi-day and potentially life-threatening flash flood event is underway across the Southwest as a strong upper-level low moves inland from California, drawing moisture from the remnants of Hurricane Polo northeastward into Arizona, New Mexico, and the southern High Plains. Moderate Risks of excessive rainfall (Level 3 of 4) are in effect from Arizona and southern Utah into New Mexico, far west Texas, and the southern High Plains through early Wednesday, with the flood threat shifting into north-central Texas and the central Plains on Wednesday. Meanwhile, a weakening nor'easter continues to drift northeast along the New England coast, bringing coastal flooding and gusty winds before exiting into the Canadian Maritimes by Wednesday. A 30.30-inch high (1028 mb) over the Great Lakes maintains quiet weather across the Midwest through Tuesday, but an approaching trough and cold front will bring showers and thunderstorms to the Great Lakes and Midwest Wednesday through Friday. Dangerous heat persists in South Texas with heat indices exceeding 105 degrees. Tropical activity spans both basins: Tropical Depression Fay and Tropical Storm Hanna in the Atlantic, Hurricane Polo (weakening) and Tropical Storm Rachel (intensifying) in the Eastern Pacific, and Hurricane Nolo in the Central Pacific. (WPC Short Range Discussion, 323 PM EDT Sep 28, 2026)
II. Surface Analysis — The Weather Map
Pressure Systems
The dominant surface feature across the central and eastern United States is a 30.30-inch (1028 mb) high pressure center holding over the Great Lakes, maintaining light winds and dry weather from the Ohio Valley through the Northeast. This high is forecast to persist through Tuesday before shifting toward the Mid-Atlantic coast on Wednesday and Thursday. (NWS Cleveland Open Lake Forecast, Lake Erie synopsis; NWS Detroit Open Lake Forecast, Lake Huron synopsis)
A 29.8-inch (1007 mb) low — the nor'easter — is positioned just southeast of Long Island, drifting northeast along the New England coast through Tuesday while filling to 29.9 inches (1015 mb). This low will dissipate over the North Atlantic on Wednesday as a 30.1-inch (1020 mb) high builds to just off the Mid-Atlantic coast. (NWS Buffalo Open Lake Forecast, Lake Ontario synopsis)
Across the West, a strong upper-level low is moving inland from California, crossing Arizona on Tuesday and reaching New Mexico by Wednesday. A 29.3-inch (992 mb) surface low is moving from the Canadian Prairies to Hudson Bay, with a 29.7-inch (1006 mb) trough building into the Plains. (NWS Marquette Open Lake Forecast, Lake Superior synopsis; WPC Short Range Discussion)
Behind the cold front, a 30.3-inch (1029 mb) high is forecast to develop over the Central Plains Thursday night into Friday, moving through the Upper Great Lakes Friday night and Saturday. A 30.4-inch (1029 mb) high will then build into the upper Great Lakes Friday and the lower Great Lakes by Saturday, before exiting into northern Maine on Sunday. (NWS Marquette, Lake Superior synopsis; NWS Buffalo, Lake Ontario synopsis)
Fronts
Slow-moving front — extends from the southern High Plains to the Upper Midwest. A moisture plume will surge northeastward along this front by Wednesday, bringing heavy rain from north-central Texas through the central Plains into Iowa, southern Minnesota, and Wisconsin. (WPC Short Range Discussion)
Cold front — dropping south from Canada into the northern Plains by Wednesday, bringing noticeably cooler air. This front will cross the upper Great Lakes Wednesday night and the lower Great Lakes Thursday night. (WPC Short Range Discussion; NWS Buffalo, Lake Ontario synopsis)
Pacific front — moving into the Pacific Northwest on Tuesday, bringing rain. (WPC Short Range Discussion)
Pressure Picture in Motion
The pressure field is in a transitional state. The Great Lakes high (30.30 inches / 1028 mb) holds firm through Tuesday but then shifts east, making way for the approaching trough from the Plains. The nor'easter (29.8 inches / 1007 mb) fills and exits. The Canadian Prairies low (29.3 inches / 992 mb) drives a cold front southward. Behind the front, high pressure rebuilds at 30.3 to 30.4 inches (1028-1029 mb), settling over the Great Lakes by the weekend. Chicago reports 30.04 inches (1017.6 mb) as of the latest observation from KORD; St. Louis reports 30.00 inches (1015.9 mb) from KCPS. (NWS observation, KORD; NWS observation, KCPS)
III. Flash Flood Event — Southwest into Southern Plains
The most significant weather event in the nation is the multi-day flash flood event across the Southwest. A strong upper-level low moving inland from California will cross Arizona on Tuesday and reach New Mexico by Wednesday. Ahead of this system, tropical moisture from the remnants of Hurricane Polo — forecast to move across Baja California tonight and into northwest Mexico on Tuesday — will be drawn northeastward. Moisture levels this high are rare this late in the season. Combined with robust lift from the upper low, they will support torrential rainfall rates and repeated rounds of storms over the same areas. (WPC Short Range Discussion)
Moderate Risks of excessive rainfall (Level 3 of 4) are in effect from Arizona and southern Utah into New Mexico, far west Texas, and the southern High Plains. Flash Flood Watches cover much of the region. Numerous instances of flash flooding are expected, and some could become life-threatening, particularly Tuesday into Tuesday night when the heaviest rain is likely to fall after dark. Burn scars, urban areas, and normally dry washes will be especially vulnerable. A few severe thunderstorms with damaging winds are also possible Tuesday across the southern High Plains. (WPC Short Range Discussion)
By Wednesday, the moisture plume will surge northeastward along the slow-moving front from the southern High Plains to the Upper Midwest. North-central Texas is a particular concern — repeated rounds of intense storms Wednesday night could produce significant flash flooding, and another Moderate Risk is in effect. Farther north, a soaking rain from the central Plains into Iowa, southern Minnesota, and Wisconsin could lead to scattered flash flooding and rising rivers. (WPC Short Range Discussion)
IV. Nor'easter — East Coast
The weakening nor'easter (29.8-inch / 1007 mb low) is positioned just southeast of Long Island, drifting northeast along the New England coast. High tides will still bring coastal flooding at times to portions of the Southeast and Mid-Atlantic coasts. Gusty northeasterly winds continue along the coast, with gusts of 40 to 60 mph reported. Notable gusts included 74 mph in Surf City, NJ, and 71 mph at Stony Brook on Long Island's North Shore early Sunday. (WPC Short Range Discussion; Yahoo News storm summary)
High pressure building in behind the storm will bring dry weather and a warming trend, with highs near 90 degrees across the Southeast by midweek, while heat indices in South Texas exceed 105 degrees. (WPC Short Range Discussion)
V. Tropical Activity
Atlantic Basin
Tropical Depression Fay — Advisory Number 32, 300 AM GMT Mon Sep 28, 2026. Located near 28.5N, 43.8W, approximately 1170 miles WSW of the Azores. Maximum sustained winds 30 mph. Moving south at 6 mph. Minimum central pressure 1012 mb (29.89 inches). No coastal watches or warnings in effect. Fay is holding on as a tropical cyclone but is expected to weaken. (NHC Public Advisory)
Tropical Storm Hanna — Advisory Number 2, 500 PM AST Mon Sep 28, 2026. Located near 36.5N, 48.7W, approximately 960 miles ENE of Bermuda and 1195 miles west of the Azores. Maximum sustained winds 45 mph. Moving east-southeast at 17 mph. Minimum central pressure 1006 mb (29.71 inches). No coastal watches or warnings in effect. Hanna is forecast to be a short-lived tropical storm, likely becoming a post-tropical cyclone by the latter part of this week. Tropical-storm-force winds extend outward up to 90 miles from the center. (NHC Public Advisory)
Eastern Pacific Basin
Hurricane Polo — Weakening. Remnants moving across Baja California, with moisture being drawn northeastward into the Southwest U.S. The moisture from Polo's remnants is contributing to the flash flood event across Arizona, New Mexico, and the southern High Plains. (WPC Short Range Discussion; NHC Polo Archive)
Tropical Storm Rachel — Advisory Number 7, 2100 UTC Mon Sep 28, 2026. Located near 14.2N, 102.1W. Maximum sustained winds 45 knots (52 mph) with gusts to 55 knots (63 mph). Moving west-northwest at 9 knots (10 mph). Minimum central pressure 999 mb (29.50 inches). No coastal watches or warnings in effect. Rachel is forecast to intensify, with maximum winds reaching 80 knots (92 mph) by 120 hours. (NHC Forecast Advisory)
Central Pacific
Hurricane Nolo — The NHC is issuing advisories for the Central Pacific on Hurricane Nolo. No additional details available from the latest NHC main page. (NHC Main Page)
VI. OPC — Atlantic and Pacific Basin Pressure Systems
Atlantic
The OPC Unified Surface Analysis (updated 28-Sep-2026 21:16 UTC) covers the Atlantic basin. Key features include the 29.8-inch (1007 mb) nor'easter low off Long Island, the 30.1-inch (1020 mb) high building off the Mid-Atlantic coast behind it, and the two Atlantic tropical systems (TD Fay at 1012 mb / 29.89 inches, and TS Hanna at 1006 mb / 29.71 inches) in the central subtropical Atlantic. The Bermuda high is not explicitly named in the latest OPC analysis, but the high pressure building behind the nor'easter will effectively serve as a ridge, bringing warming temperatures to the Southeast. (OPC Unified Surface Analysis; OP