Valid Thursday, February 28, 2019 to Monday, March 11, 2019

Summary of Forecasts and Hazards

US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT May 22 2019

Synopsis: tbd

Hazards

• n/a

Detailed Summary

For Saturday May 25 - Wednesday May 29: WPC Days 3-7 U.S. Hazards

For Thursday May 30 - Wednesday June 05: Model differences give some uncertainty to the 500hPa height pattern that will dictate the week-2 forecast – the GEFS has further deamplified the mean ridge over the Southeast for days 8-14, while the ECMWF has shifted the troughing over the Great Lakes northward over Canada, allowing the positive 500hPa height anomalies over the Southeast to spread into the Tennessee Valley. As this ridge starts to deamplify down mid-week, the ridge remains over the Gulf Coast in the ECMWF, while the GEFS pushes the ridge further west over Texas. Both indicate more zonal flow for the latter half of the period. This ridge placement has caused differences in the forecast for heat in the Southeast from the two models. The ECMWF solution has a much longer heat event, and as zonal flow takes over mid-week, dew points start to rise and play more of a role in the risk for excessive heat. The GEFS is not quite as warm as the ECMWF but does indicate a possible second spike in heat anomalies mid-week from the probabilistic tool. Due to the consistency in this ECMWF solution, it is favored. The high risk for excessive heat over southern Alabama, Georgia and South Carolina has been extended from yesterday's forecast, valid from May 30 – Jun 2, taking the influence humidity is likely to start having mid-period. A broader moderate risk is forecast until June 3rd through the Deep South, while a slight risk, also through June 3rd, is extended through parts of the Mid-Atlantic and central Gulf Coast. With the lack of precipitation over the next two weeks and the duration of this heat event, there is a possibility of flash drought development in parts of the region, especially in the high and moderate risk regions.

This classic ridge-trough pattern is likely to set up a mean baroclinic zone aligning from the central and southern Plains through the western Great Lakes through most of week-2. This is expected to be a fairly stationary feature throughout the week-2 period so a slight risk of heavy precipitation is forecast for the week. Both the ECMWF and GEFS probabilistic tools indicate a 20% chance of 2 day rainfall totals reaching 1 inch through the entire period. A strong surface cyclone is expected to develop along this baroclinic zone prior to the period, lasting into day 8 (May 29) before lifting northeastward over the Great Lakes. Model guidance is showing better agreement on today's 06z runs for a second storm system to move through mid-period (5/31-6/1), again leading to a period of enhanced rainfall. A moderate risk of heavy precipitation is posted over the region likely to be most impacted by these surface systems and where the models have indicated a 40% chance of rainfall amounts over 1 inch, from northeastern Texas northward toward western Illinois, for the aforementioned days. Much of the region highlighted for both the slight and moderate precipitation risks is currently at 95-99 percentile for soil moisture with the anomalously high amount of rainfall they have received this spring. River and flash flooding will likely be easily triggered due to current conditions.

Upper-level ridging over mainland Alaska is forecast to persist through most of the period, though weak. An upper-level trough over the Aleutians will likely lead to fairly active weather throughout the Gulf of Alaska, there are no hazards forecast for the state.

Forecaster: Christina Maurin

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