ENHANCED SEVERE WEATHER RISK / SUBTROPICAL DEPRESSION ONE FORECAST SYNOPSIS ISSUED APR. 19, 2017…4:05 P.M. EDT

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(T. F. “Storm” Walsh III)

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Good day everyone!

The SPC has designated the following severe weather risk areas in the Day 1 Outlook as of the 1630Z issuance:

…THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON/EVENING FOR SOUTHEAST NEBRASKA AND PARTS OF WESTERN/CENTRAL IOWA…

…THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS SURROUNDING THE ENHANCED RISK AREA…FROM SOUTHWEST WISCONSIN TO NORTHEAST KANSAS…

…THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE SLIGHT RISK AREA…FROM NORTHERN OKLAHOMA TO LOWER MICHIGAN AND NORTHERN OHIO…

…SPC SUMMARY…
Severe thunderstorms capable of producing large hail, damaging winds, and a few tornadoes will be possible from the central Plains to the upper Mississippi Valley mainly from mid-afternoon into tonight.

SPC DAY 1 CONVECTIVE OUTLOOK

Probability of one inch diameter hail or larger within 25 miles of a point.
Hatched Area: 10% or greater probability of two inch diameter hail or larger within 25 miles of a point.

Based on the analysis of the outlook,severe thunderstorms capable of producing all severe threats will be ongoing from this afternoon into tonight across the risk areas, mainly the enhanced and slight risk areas.  Based on analysis of some forecast soundings for the region,mainly the SWEAT Index, and various tornado forecast parameters, the RAP model forecast soundings, initialized 12Z today, tends to indicate the most likely areas to experience the worst of the severe threats, including isolated tornadoes, to lie within the outlined areas on the F5 DATA maps.  The RAP model indicates the best probability to occur from around 5:00 p.m. to 8:00 p.m. CDT.  Thereafter, the modeling suggests activity should begin to weaken somewhat as it moves eastward.

F5 DATA RAP MODEL SOLUTION 5:00 P.M. AND 8:00 P.M. CDT

Residents in the outlined SPC risk areas, should monitor NOAA Weather Radio, and local NWS statements for up to date information regarding any necessary actions.

IF A TORNADO WARNING IS ISSUED FOR YOUR AREA…SEEK SHELTER IMMEDIATELY!

The following graphics are linked to their respective sites.  Mouse over and click on the graphics for up to date information.

SPC CONVECTIVE WATCH DISPLAY

SPC MESOSCALE DISCUSSIONS DISPLAY

NWS DOPPLER RADAR SITE MAP

INTELLICAST RADAR SUMMARY (Click local area once map displays to zoom in)

NWS HAZARDS AND WARNINGS DISPLAY (click on graphic, then your area of interest)

SUB-TROPICAL DEPRESSION ONE:

Well, another surprise.  INVEST 91L was designated Sub-tropical Depression ONE earlier today by the NHC.  As of the 11:00 a.m. EDT advisory, the following information was available on ST Depression ONE:

NHC WEBSITE LINK:
http://www.nhc.noaa.gov/

11:00 AM AST Wed Apr 19
Location: 31.9°N 40.9°W
Moving: NNE at 8 mph
Min pressure: 995 mb / 29.38 in.
Max sustained: 35 mph

SUBTROPICAL DEPRESSION ONE SATELLITE FLOATER LOOP

HURREVAC NHC TRACKING MAP S.T. DEPRESSION ONE:

Maximum sustained winds were 35 mph, mainly away from the center in the convective band to the SE of the center.

Based on analysis of SST’s and wind shear, I concur the depression should not strengthen, and should begin to weaken once absorbed by the large, extratropical low.

Based on current steering, and NHC data, STD ONE is moving to the NNE.  Satellite loop imagery indicates the exposed center of the depression has been meandering today, making a cyclonic loop.  Based on forecast steering layers maps, valid 00Z this evening, another semi-cyclonic loop is forecast during the next 48 hours. This would be the beginning of the forecast NNW motion by the NHC as the depression becomes affected by a large, extratropical low, in which S.T. Depression ONE will be absorbed by.  Once this occurs, a motion off the the E to ENE should ensue.

Based on earlier information, and limited analysis on my part due to time, it did not appear likely that 91L would attain STD status.  One is probably wondering how this came to be.  When I arrived home, and saw we had the depression, I engaged in some further in depth analysis and research.  Criteria set forth dictates SST’s for Sub-tropical development should be at least 23C (73F).  A recent article I found, suggested that 21C is sufficient, along with the explanation of the difference between a Tropical and Subtropical system:

Tropical, subtropical, extratropical?

It is often difficult to tell from looking at forecast model data whether a low that is expected to develop near the U.S. coast will be tropical, subtropical, or extratropical. The difference is important, since tropical systems have the potential to quickly grow into hurricanes, while extratropical or subtropical storms do not. So, here’s a quick meteorology lesson on the normal progression one sees from extratropical cyclone, to subtropical cyclone, to tropical cyclone.

  1. An extratropical cyclone forms. Extratropical cyclones have cold air at their core, and derive their energy from the release of potential energy when cold and warm air masses interact. These storms always have one or more fronts connected to them, and can occur over land or ocean. An extratropical cyclone can have winds as weak as a tropical depression, or as strong as a hurricane. Examples of extratropical cyclones include blizzards, Nor’easters, and the ordinary low pressure systems that give the continents at mid-latitudes much of their precipitation.
  2. If the waters under the extratropical cyclone are at least 21C (70F), thunderstorm activity will gradually build inside the storm and moisten and warm the lower levels. Over time, the core of the storm may gradually go from cold to warm, and the storm will start getting some of its energy from “latent heat”, which is the energy released when water vapor that has evaporated from warm ocean waters condenses into liquid water. Latent heat is what powers tropical cyclones. At this point, the storm is called subtropical. If the winds are already more than 39 mph (as happened in the case 2007’s Subtropical Storm Andrea), it is called a subtropical storm. If the winds are less than 39 mph, then it is called a subtropical depression. So, you don’t need to start with a subtropical depression in order to get a subtropical storm.
  3. If the subtropical storm remains over warm water for several days, it may eventually become fully tropical, and be called a tropical storm. This happens when thunderstorm activity starts building close to the center of circulation, and the strongest winds and rain are no longer in a band far from the center. The core of the storm becomes warm, and the cyclone derives all of its energy from the “latent heat” released when water vapor that has evaporated from warm ocean waters condenses into liquid water. One does not find warm fronts or cold fronts associated with a tropical cyclone.

A subtropical storm typically has a large, cloud free center of circulation, with very heavy thunderstorm activity in a band removed at least 100 miles from the center. The difference between a subtropical storm and a tropical storm is not that important as far as the winds they can generate, but tropical storms generate more rain. There is no such thing as a subtropical hurricane. If a subtropical storm intensifies enough to have hurricane force winds, then it must have become fully tropical. The definition of a subtropical storm, according to the National Hurricane Center in 2005:

A non-frontal low pressure system that has characteristics of both tropical and extratropical cyclones. The most common type is an upper-level cold low with circulation extending to the surface layer and maximum sustained winds generally occurring at a radius of about 100 miles or more from the center. In comparison to tropical cyclones, such systems have a relatively broad zone of maximum winds that is located farther from the center, and typically have a less symmetric wind field and distribution of convection.

A second type of subtropical cyclone is a mesoscale low originating in or near a frontolyzing zone of horizontal wind shear, with radius of maximum sustained winds generally less than 30 miles. The entire circulation may initially have a diameter of less than 100 miles. These generally short-lived systems may be either cold core or warm core.

So, based on analysis of some of the thermodynamics involved, albeit SST’s are only 21C, what has most likely occurred is, the surrounding atmosphere is cool enough in relation to the SST’s (Sea Surface Temperatures), that thunderstorms (convection) was able to form, which in turn warmed and moistened the lower atmosphere, releasing latent heat.  In fact, I had mentioned the other day that this system was clod core up to around the 400 mb level.  This afternoons analysis of AMSU data from CIMMS, indicate the system is now warm core.  In addition, the atmosphere over the surface circulation is pretty cold as you go up with height.  Analysis at approximately 550 mb indicates the atmosphere is approximately-2.2F to -4.0F, at 350 mb -32.8F, and 200 mb -52.6F.  These differences in  the colder environment, are pretty much what happens in the Tropics for a tropical system to develop, in a much warmer environment with the needed 80F SST criteria.

AMSU THERMAL DATA

AMSU 550 MB THERMAL DATA

AMSU 350 MB THERMAL DATA

AMSU 200 MB THERMAL DATA

Elsewhere, Tropical Storm formation is not expected.

Have a blessed day!

T. F. “STORM” WALSH III
GMCS, USCG (ret)
METEOROLOGIST / HURRICANE SPECIALIST /SEVERE WEATHER SPECIALIST
MEMBER WEST CENTRAL FLORIDA AMS
CoCoRAHS OBSERVER

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About palmharborforecastcenter

I am a Tropical Forecast meteorologist, providing hurricane forecasts during the Atlantic Hurricane Season. I retired from the U.S. Coast Guard in July of 2001. Meteorology became my passion in high school, and I have continued my educational background in meteorology since 1996, when I undertook the study of Tropical Meteorology. While working toward my degree, I had to unexpectedly withdraw from college due to my oldest sons medical reasons. I do however, meet the educational criteria of the AMS to be recognized as a meteorologist. Studies include, but are not limited to the Navy Aerographers Mate course, Naval METOC meteorology course, Meteorology 2010 Sophomore level course while attending St. Petersburg College, Clearwater, FL., Basic Forecasting course for operational meteorologists from Rapid WX, meteorology institute, a four month meteorological internship, and extensive research on numerous meteorological topics such as the MJO, NAO, satellite imagery interpretation, etc. I have been forecasting Tropical Weather (Tropical Storms and Hurricanes) since 1996, with my main client being three different Coast Guard Commands.
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7 Responses to ENHANCED SEVERE WEATHER RISK / SUBTROPICAL DEPRESSION ONE FORECAST SYNOPSIS ISSUED APR. 19, 2017…4:05 P.M. EDT

  1. dellamom says:

    I see we have T.S. Arlene, folks. Prayers to ships at sea who may be impacted by winds. The 6:36 a.m. EDT Atlantic Tropical Weather Discussion states: “Gale-force winds have developed south of a 990 mb surface low located southwest of Arlene.”

  2. Monty says:

    Thanks Storm. Not good Dellamom sure we have the same issues in Houston as does Storm and everyone. Storm…hope all went well at docs!!

  3. dellamom says:

    Not June. Not even May. Please don’t tell the insurance companies because they will start calculating hurricane season for flood insurance purposes after March if they decide this is a trend. 😦 Thank you, Storm, for staying on top of this. No one I’ve spoken with knew there was anything going on at all, much less a subtropical depression. We know because we have you. I know this is during your work week and appreciate the extra time and effort it took to analyze this and explain it for us. God bless you and all ships at sea.

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