Wednesday, September 20, 2017 5:40 AM

Warm Weather To Continue Through Work Week, But Wet Weekend Ahead

Storms That Go Bump in the Night

Severe thunderstorms broke out yesterday afternoon across eastern North Dakota and northeastern South Dakota. There were a couple of tornado touchdowns in South Dakota, but the storms mainly produced large hail and straight line damaging winds. The line of storms has made slow progress eastward since the individual storms were moving more south-to-north than west to east (see infrared loop from College of DuPage Satellite and Radar Menu). During the early morning hours, the storms moved into central Minnesota, bringing briefly heavy rainfall, wind gusts of 50-60 MPH and some hail (see NWS Upper Mississippi Valley radar loop). At the St. Cloud Regional Airport, the storms produced 0.56 inch of rain, and wind gusts to 51 MPH. At St. Cloud State University, I measured 1.08 inch of rain. There was also some occasional pea-sized hail at my home.

All of the storms are being produced as a front is pushing eastward from the Dakotas into Minnesota (see 24-hour loop of NWS surface maps). The large number of thunderstorms is being triggered by the combination of fairly deep humid air (dew points in the 60's on the UCAR hourly dew point chart with plenty of moisture more than a mile upwards), strong winds aloft, and an upper-level low pressure system pushing through the Dakotas (see water vapor loop from College of DuPage Satellite and Radar Menu).

Drying Out But Still Warm Today

As the front pushes eastward through Minnesota, the storms will continue through the state during the pre-dawn hours. However, they will likely weaken around sunrise. We may have some lingering morning clouds, but sunshine should return this afternoon. Since the air behind this front isn't much cooler than what we had yesterday. However, the air behind this front is much drier with dew points in the comfortable 50's and even the 40's in the western Dakotas and Nebraska (see UCAR hourly dew point chart). So, our high temperatures will climb into the 70's thanks to more sunshine than yesterday.

The drier air will combine with clear skies to allow low temperatures tonight to drop to the lower 50's or even the upper 40's.

Return of 80's Thursday

Tomorrow, the next surge of warmer air will begin to push northeastward. Since the ground air will be drier, we should get through tomorrow rain-free. High temperatures will climb to near or even above 80 degrees. You'll notice the increase of humidity as the day wears on.

Some Storms Thursday Night, July Relapse Friday

The warmer and more humid air will push back into Minnesota by Friday. On the way up, we will have a good chance for showers and thunderstorms Thursday night. Some large hail is possible in these storms.

We should break out into the warmer air on Friday. As long as skies clear, we could see highs well up into the 80's and possibly see highs approaching 90 degrees. The perfect way to celebrate the autumnal equinox.

Stormy Weekend Ahead With Heavy Rainfall Problems in Spots

The weather pattern is expected to become very slow moving late in the week into the weekend. The West will continue to be dominated by the low pressure area seen now in the Pacific Northwest (see infrared loop from College of DuPage Satellite and Radar Menu). Meanwhile, a high pressure area will stall over the eastern US. Unfortunately, this means that the front separating cooler air over the Rockies from the very warm and humid air we will see on Friday will have a tough time making eastward progress. That could leave Minnesota, Iowa, Nebraska, Kansas, and Oklahoma under the gun for a series of showers and thunderstorms. There might be a threat of severe weather Friday night with these storms, but the most persistent problem will be heavy rain as storms develop each day and move through about the same area. While this rain would be welcome in the Dakotas to northwestern Minnesota, it might be too much rain further to the south and east.

And, of course, the timing of the rain couldn't be worse with periods of showers and thunderstorms Friday night through at least Monday. All of the clouds will keep high temperatures down over the weekend. On Saturday, we could see highs limited to the lower 70's, but the humid conditions will continue. Sunday, we could be on the cooler side of the front with lots of clouds and periods of rain. Highs will be kept in the 60's.

Look for painfully slow improvement early next week. The rain will eventually get lighter and less persistent beginning on Sunday with drying trend into next week. However, we may not see much sunshine until weather systems get moving again, and that could take until the middle of next week.

Hurricane Maria Clobbering Virgin Islands, Headed for Puerto Rico Today, Dominican Republic Tonight

Hurricane Maria has been moving northwestward through the northeastern Caribbean Sea overnight (see infrared Atlantic tropical satellite loop from Penn State ewall) and the strongest part of the storm is moving through the Virgin Islands with a direct hit on St. Croix and is approaching Puerto Rico (see infrared zoom-in loop from University of Wisconsin SSEC). Maria has top winds of 165 MPH, but more importantly, the hurricane force winds are in a wider band, now extending 35-50 miles from the center, thanks to a second band of intense thunderstorms surrounding the eye. Tropical storm force winds extend 100-150 miles from the center. The storm will have a huge impact on the already damaged Virgin Islands as well as Puerto Rico, although the center is wobbling a bit to the southeast and east of Puerto Rico right now, but the National Hurricane Center forecasters are still expecting major impacts on the island. The mountains of Puerto Rico and Hispanola as well as stronger winds aloft are expected to weaken Maria after landfall in Puerto Rico this morning. Still, the storm is expected to continue being a major hurricane as it moves northwestward. Any strike in the Dominican Republic and Haiti will be catastrophic due to the poverty in those countries. The storm could affect parts of the Bahamas, Turks, and Caicos by Friday, but is expected to move to the northeast of the Bahamas, lessening the impact.

Maria has already had a huge impact on Dominica, Guadeloupe, and St. John Island with much still unknown due to power losses on those islands. Again, it has passed directly over St. Croix and will greatly affect the already damaged Virgin Islands in addition to Puerto Rico.

At this point, the computer forecasts mostly have the storm paralleling the East Coast in the Atlantic from the weekend into next week. There are a few of the computer forecasts that have Maria passing close enough to parts of the East Coast to bring some rain and wind on land. All of those forecasts are too far out for any certainty, so the storm will have to be watched. However, it does appear that Florida and Texas, which took the brunt of earlier storms, will escape Maria.

Outer Bands of Jose Affecting New England

Meanwhile, Tropical Storm Jose (at the eastern edge of the infrared loop from College of DuPage Satellite and Radar Menu) has brought some of its outer rain bands into southeastern New England (see NWS Northeast radar loop). There are also some fairly strong winds on Long Island and the islands off southeastern Massachusetts. Top wind gusts are 70 MPH out at sea, but a tropical storm warning is in effect for extreme southeast New England. The northeastward progress of Jose is expected to be blocked, so the storm will likely meander off shore for a while. The winds should slowly reduce as the storm is over cooler ocean water. Still, Jose is a large storm, so the large area of strong winds could bring minor coastal flooding from Delaware to Massachusetts. If the storm drifts only a bit further to the west, stronger winds and rain could affect the Long Island, southern New England, and New Jersey coasts.

Confidence Level: "I Can Turn on the Computer"

Wednesday 9/20/2017: A cloudy start, then becoming sunny, breezy, turning less humid, and still warm. High: between 72 and 76. Winds: W 10-20 MPH. Chance of measurable rainfall: 10%

Wednesday Night: Clear, light winds, and cool. Low: between 47 and 52. Winds: light winds early, becoming SE 5-15 MPH late. Chance of measurable rainfall: 10%.

Thursday 9/21/2017: Sunny, breezy, much warmer, and becoming noticeably humid as the day goes on. High: between 78 and 83. Winds: SE 10-20 MPH. Chance of measurable rainfall: 10%

Thursday Night: Partly to mostly cloudy, breezy, warmer, and turning uncomfortably humid. A good chance of a shower or thunderstorm. Some storms could contain hail. Low: between 64 and 68. Winds: S 8-18 MPH. Chance of measurable rainfall: 50%.

Friday 9/22/2017: Perhaps an early thunderstorm, then becoming partly sunny, breezy, very warm and uncomfortably humid. A chance of late day showers and thunderstorms. High: between 85 and 90. Winds: S 15-25 MPH. Chance of measurable rainfall: 40%

Confidence Level: "I Can Remember Where I Wrote Down the Password and Type It In Correctly"

Friday Night: Showers and thunderstorms likely. Heavy rainfall and severe weather possible. Low: between 64 and 68. Winds: SE 15-25 MPH with higher gusts in storms. Chance of measurable rainfall: 70%.

Saturday 9/23/2017: Lots of clouds, not as warm, but still sticky with showers and thunderstorms likely. Heavy rainfall possible. High: between 70 and 75. Winds: E-NE 8-15 MPH. Chance of measurable rainfall: 80%

Confidence Level: "After 5 Minutes of Boot Up Time, I Can Remember Why I Turned on the Computer"

Saturday Night: Cloudy with occasional rain or shower. Heavy rainfall and severe weather possible. Low: between 60 and 65. Winds: NE 5-10 MPH. Chance of measurable rainfall: 60%.

Sunday 9/24/2017: Cloudy and a bit cooler, but still humid. Periods of rain showers. High: between 62 and 68. Winds: NW 8-15 MPH. Chance of measurable rainfall: 60%

Extended: It may not dry out until Tuesday.

Forecast Confidence (10 - "Will more of your hair fall out, Bob?"; 0 - "Will the winning PowerBall Numbers be encoded in your lost hair, Bob?"): 7 Wednesday through Thursday, 6 Thursday night and Friday, 5 Saturday, 3 Saturday night and Sunday, 1 Monday and Tuesday.

Yesterday's High: 74°F; Overnight Low (through 5 AM): 62°F
St. Cloud Airport Rainfall (through 5 AM Wednesday): 0.56 inch; SCSU Precipitation (through 5 AM Wednesday): 1.08 inch

Next Update: Friday, September 22, 2017 8 AM (or as needed) No Forecast Thursday due to Rosh Hashanah

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Let me know what you think about this forecast and discussion by emailing SCSU meteorology professor Bob Weisman. Please note that I make the forecast, not the weather!

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