Tuesday, September 19, 2017 4:40 AM

Foggy Start, Stormy Finish

Cloudy, Humid, But Not Much Warmer Today

The cooler, drier air mass allowed sunshine on Sunday, but the front hung around to our south yesterday (see 24-hour loop of NWS surface maps). That kept the clouds in most of the day yesterday (see infrared loop from College of DuPage Satellite and Radar Menu). There were some sprinkles or light showers spread out during the day (see NWS: Last 72 hours of St. Cloud Observations). The persistent clouds and the cool start limited high temperatures to the upper 50's in much of eastern Minnesota yesterday (see 3-day loop of high temperatures from NWS/University at Albany). Overnight, the high clouds left the area, but much higher dew points to our south (upper 50's in Minnesota, but middle to upper 60's in Iowa on the UCAR hourly dew point chart) has produced widespread low clouds and fog (see Colorado State RAMDIS Western US Fog/Reflectivity Product) across Minnesota. Because of the persistent southeast winds (see UCAR Minnesota surface chart), the visibility is generally over a mile, but these low clouds are quite thick.

Today and tonight, the warmer and stickier air well to the south of Iowa will try to climb over the top of the cooler air at the ground. This morning, this process will trap the fog and low clouds over Minnesota, so I am not optimistic about a lot of sunshine. The humidity level will feel like summer, but temperatures won't climb that much due to the clouds. Highs will generally be in the upper 60's to perhaps low 70's if we can get an afternoon break in the clouds.

Storms Likely This Evening, With Heavy Downpours, Damaging Winds Possible

Overnight tonight, one of the low pressure areas in the strong counterclockwise circulation that is dominating southwestern Canada and the Pacific Northwest (see infrared loop from College of DuPage Satellite and Radar Menu) will pull its front eastward from Montana (see NWS WPC Latest North American zoom-in surface map). This lead low, that pushed from Idaho and Montana into Alberta overnight (see water vapor loop from College of DuPage Satellite and Radar Menu), will also have some very strong winds aloft helping to produce the lift. This will produce a batch of strong thunderstorms this afternoon in the eastern Dakotas, which will push into Minnesota overnight. Because of the really humid air and the strong winds aloft, any decent amount of sunshine in the Dakotas will produce a threat of all kinds of severe weather, including the chance for tornadoes. There is the possibility of some heavy downpours with these storms, which will arrive in central Minnesota most likely between 6 PM and midnight. I have only a 1 in 5 chance of afternoon thunderstorms, then thunderstorms likely between 6 PM and early morning. There might be a scattered late night shower or thunderstorm.

Drying Out Tomorrow and Thursday

Since the air behind this cold front will actually be pulled down the eastern slopes of the Northern Rockies, temperatures won't get that much cooler tomorrow. We will have a slow drying out with uncomfortable morning humidity, but most comfortable afternoon conditions. We might have a break or two of sunshine, but the upper level low now pushing through Oregon (see water vapor loop from College of DuPage Satellite and Radar Menu) will produce a lot of afternoon clouds. There may even be a slight chance of a few brief showers in north central and northeastern Minnesota. Despite the cooler air mass, tomorrow could be warmer than yesterday and today because of the drier air and the thinning clouds.

We should dry out enough for skies to clear tomorrow night. Low temperatures will drop back into the 50's. However, by late at night, the next wave of warmer air will begin to move over the cooler air near the ground. That will limit how cool we get.

80's Back for the Equinox

On Thursday and Friday, we should again see a major warm-up. If we can manage more sunshine, high temperatures on Thursday will make a strong run at 80 degrees. The heat pump will continue Thursday night, raising the chance for showers and thunderstorms. Some storms could produce some large hail, but it's too early to be sure how big the threat will be. If we get rid of any lingering early showers, Friday, the day of the autumnal equinox, will feel like mid-July with highs well up into the 80's and uncomfortable humidity. (note that meteorological fall began September 1 as these three months, Sept., Oct., and Nov., mark the transition between summer and winter)

More Storms Into The Weekend??

A cold front will push into Minnesota on Friday night, raising the chance for showers and thunderstorms. Thanks to the flow pattern over North America (big low near the West Coast, strong high in the eastern states; see water vapor loop from the College of DuPage Satellite and Radar Menu), Minnesota could see the front stall out over us or near us. That would set the stage for a very humid Saturday, but with chances of thunderstorms. It's much too early to pin down where the best chances for heavy rainfall and severe weather may be.

Hurricane Maria Pounding Caribbean Islands Today, Threat to Virgin Islands, Puerto Rico By Tomorrow

Hurricane Maria pounded the middle Leeward Islands yesterday (see infrared Atlantic tropical satellite loop from Penn State ewall) as it intensified. Top winds are now 155 MPH, making the storm another category 5 system in the Atlantic basin. Maria had 160 MPH top winds as it moved over Martinique and Dominica in the middle of the night. Contact with these islands is limited due to the storm, but these islands will have similar damage to what we saw from Irma over Barbuda, Antigua, and the Virgin Islands. The only good part of Maria is that the eye wall is more narrow than in Irma, so the hurricane force winds are limited to within 20-25 miles of the storm.

Conditions will remain favorable (very warm water, little change in the wind with height) for Maria to keep its top winds of at least 125 miles per hour over the next couple of days, so it will hit the British and US Virgin Islands hard, especially since there were major impacts from Irma on these same islands. Puerto Rico is also under a hurricane warning with major impact likely on part or all of the island. Mudslides are also possible.

Maria is forecast to continue its northwestward track, but that path becomes more uncertain after the storm passes Puerto Rico. Note that the white hatched cone of uncertainty (storm center could be anywhere within this area) gets much bigger after a forecast landfall on Puerto Rico tomorrow. Maria could strike Hispanola, Cuba, or the Bahamas, but later forecasts will clear this up. It is still quite uncertain whether Maria poses a threat to the US mainland. I have no more confidence in the current long-range forecasts keeping it out to sea next week than I did when it was forecast to strike the Carolinas or Georgia last week. Still, Maria will be a destructive storm in the Caribbean.

Jose to Bring Some Stiff Winds, Rain to Northeast Coast

At the northern fringe of the infrared Atlantic tropical satellite loop is Hurricane Jose well to the east of the North Carolina coast (better view on the Northeast infrared satellite loop from College of DuPage Satellite and Radar Menu). Top winds are 75 MPH. The storm is large, but isn't as well organized as Maria since most of the thunderstorms are off on the northern and western flank of Jose (compare with Maria which has a drier area but more thunderstorms surrounding the storm on all sides) . Jose is expected to weaken a bit and perhaps become more of a middle-latitude storm in the next couple of days. However, it is large enough and strong enough to produce swells along the East Coast, Bermuda, and the Bahamas. As the storm moves northeastward, it is likely to produce tropical storm force winds over parts of New England and Long Island. Also, the thunderstorms on the western flank are beginning to approach the East Coast, so rains are likely from Delaware to Massachusetts with heavier rain in Connecticut, Rhode Island, and Massachusetts. The storm is expected to stall off the coast of southeastern Massachusetts later in the week. It won't have 70+ MPH winds by then, but could easily have 40-50 MPH winds. Where it goes is uncertain.

Ingredients for Active Atlantic Tropical Cyclone Season Still There

What makes an Atlantic Hurricane season so active? There are and have been a large number of storms crossing Africa near the Equator, although the series has more of a break in western Africa. When these storms pass over the tropical Atlantic, they can strengthen over the very warm water of the tropical Atlantic. At the start of the season, most of the western tropical Atlantic south of the Carolinas had water temperatures over 30°C (86°F). Irma has churned the water in part of the Atlantic approaching the Leeward Islands, so water temperatures are in the lower 80's (28-29°C), but there is still plenty of mid-80's water (brown) available. And, the minimum for hurricane development is 26-27°C (about 80°F) Once a storm can tap that warm water, all it needs is light winds aloft to create a hurricane. Now, former tropical storm Lee hit a patch of strong winds aloft and weakened. Still, the ingredients remain in the Tropical Atlantic for more storms.

Confidence Level: "I Can Turn on the Computer"

Tuesday 9/19/2017: Fog early, then mostly cloudy, warmer, and becoming uncomfortably humid. A slight chance of a late day shower or thunderstorm. High: between 68 and 72. Winds: SE 15-25 MPH. Chance of measurable rainfall: 30%.

Tuesday Night: Showers and thunderstorms likely, breezy, warm and sticky. Some storms could contain heavy downpours and severe weather, with the biggest threat being straight-line damaging winds. Low: between 62 and 66. Winds: SE 15-25 MPH, becoming NW 10-20 MPH late. Higher gusts possible in thunderstorms. Chance of measurable rainfall: 70%.

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

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

Wednesday Night: Partly clear, light winds, and cool. Low: between 50 and 55. Winds: light winds early, becoming SE 8-15 MPH late. Chance of measurable rainfall: 10%.

Thursday 9/21/2017: Sunny, breezy, and much warmer. High: between 78 and 83. Winds: SE 10-20 MPH. Chance of measurable rainfall: 10%

Thursday Night: Partly clear, 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%.

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

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 84 and 88. Winds: S 15-25 MPH. Chance of measurable rainfall: 40%

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 occasional showers and thunderstorms. High: between 70 and 75. Winds: S 15-25 MPH. Chance of measurable rainfall: 10%

Extended: Another warm and sticky night Friday night?? Still humid with chances for storms over the weekend???

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 Tuesday, 6 Tuesday night, 5 Wednesday through Thursday, 4 Thursday night, 3 Friday, 2 Saturday.

Yesterday's High: 59°F; Overnight Low (through 4 AM): 54°F
St. Cloud Airport Rainfall (through 4 AM Tuesday): 0.02 inch; SCSU Precipitation (through 4 AM Tuesday): TBA

Next Update: Wednesday, September 20, 2017 8 AM (or as needed)

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