In Brief:

Some mid level moisture, summer heat, and passing short waves (+ some up slope moisture supply) means we’ll have afternoon thunderstorms Wednesday to Friday. Wednesday is the peak of this activity with a Marginal Risk of severe weather painted for the Front Range.

3pm Update:

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URGENT - IMMEDIATE BROADCAST REQUESTED
   Severe Thunderstorm Watch Number 611 (See Figure 2 update)
   NWS Storm Prediction Center Norman OK
   245 PM MDT Wed Aug 21 2019

   The NWS Storm Prediction Center has issued a

   * Severe Thunderstorm Watch for portions of 
     Eastern Colorado
     Southeast Wyoming

   * Effective this Wednesday afternoon and evening from 245 PM
     until 900 PM MDT.

   * Primary threats include...
     Scattered large hail and isolated very large hail events to 2
       inches in diameter possible
     Isolated damaging wind gusts to 70 mph possible

   SUMMARY...Isolated severe storms, in the form of sustained
   multicells and a few transient supercells, are expected to develop
   along the foothills/I-25 corridor of eastern Colorado into southeast
   Wyoming. Isolated large hail will be the primary risk as storms
   drift east-southeastward through late afternoon/evening, although
   localized severe-caliber winds could also occur, particularly across
   east-central Colorado.

   The severe thunderstorm watch area is approximately along and 45
   statute miles east and west of a line from 75 miles north northeast
   of Laramie WY to 30 miles southeast of Pueblo CO. For a complete
   depiction of the watch see the associated watch outline update
   (WOUS64 KWNS WOU1).

   PRECAUTIONARY/PREPAREDNESS ACTIONS...

   REMEMBER...A Severe Thunderstorm Watch means conditions are
   favorable for severe thunderstorms in and close to the watch area.
   Persons in these areas should be on the lookout for threatening
   weather conditions and listen for later statements and possible
   warnings. Severe thunderstorms can and occasionally do produce
   tornadoes.

   &&

   OTHER WATCH INFORMATION...CONTINUE...WW 609...WW 610...

   AVIATION...A few severe thunderstorms with hail surface and aloft to
   2 inches. Extreme turbulence and surface wind gusts to 60 knots. A
   few cumulonimbi with maximum tops to 450. Mean storm motion vector
   32020.

   ...Guyer




Figure 2 update: SPC severe thunderstorm watch from the offices in Norman OK early afternoon Wednesday.

End 3pm Update.

1:30pm Update:

I’m expecting a watch will be issued soon, but I’ll let the SPC do the talking for themselves:

Mesoscale Discussion 1826
   NWS Storm Prediction Center Norman OK
   0151 PM CDT Wed Aug 21 2019

   Areas affected...The Colorado front range to portions of southeast
   Wyoming

   Concerning...Severe potential...Watch possible 

   Valid 211851Z - 212045Z

   Probability of Watch Issuance...40 percent

   SUMMARY...Thunderstorms will develop/intensify through early
   afternoon and pose a threat for severe hail and brief severe wind
   gusts.  The potential coverage of the threat, and hence the need for
   a Severe Thunderstorm Watch, is uncertain.

   DISCUSSION...Strong heating and an approaching weak mid-upper-level
   low are contributing to deepening cumulus and initial thunderstorm
   development over the Colorado and Wyoming high terrain.  A moist and
   modifying continental polar air mass has pushed up to the foothills,
   creating an environment supportive of thunderstorms that varies
   little with latitude from the Raton Mesa northward to the Laramie
   Mountains.  By early afternoon, forecast soundings suggest this
   environment will consist of 1500-2500 J/kg of MLCAPE (higher toward
   the Mountains) with little convective inhibition, moderate
   straight-line vertical wind shear confined mostly in the 1-4 km
   layer (supportive of multicell to some transient supercell
   structures), and DCAPE of 1200-1500 J/kg.  

   Given storm rotation and steep low-to-mid level lapse rates, severe
   hail is the main threat with these storms, but brief severe wind
   gusts are possible in the stronger downdrafts.  Moderate low-level
   southeasterly upslope flow combined with relatively weak
   west-northwesterly to westerly flow at 500-300 mb suggests a slow
   south to southeast motion to the cells, thus limiting the eastward
   extent of the threat.  The main uncertainty concerning the need for
   a Severe Thunderstorm Watch is the coverage of the hail/wind threat.
    The coverage of storms that mature and move into the foothills will
   be monitored in the next hour or so to better determine if a Watch
   will be needed.

   ..Coniglio/Guyer.. 08/21/2019

   ...Please see www.spc.noaa.gov for graphic product... (See Figure 1 update). 
Figure 1 update: SPC mesoscale discussion from Norman OK early afternoon Wednesday. The pink dot is Longmont, CO.

End 1:30pm Update.

Forecast Discussion:

For Wednesday, even though there is a trough to the east and west of the state, the moisture is having to take quite a trip to get here. The low level moisture on the Plains is slipping back on easterly surface winds – pushed by the big thunderstorm complex that has traveled to the northern Midwestern states in the images below. The mid level moisture is almost taking a Hawaiian vacation before getting here (Figure 1). Our usual satellite image looks like the moisture is all cut off from the south (Figure 2) but Figure 3 is the same image with a different color scale. There is “some” moisture around from Texas and New Mexico up to us.

The Marginal Risk (for hail and high winds) is along this quasi-cold front created by the traveling thunderstorm masses way out east. It includes all of eastern Colorado up into the foothills.

The Longer Range Forecast:

For Thursday the west coast trough sneaks a closer and interacts with the remaining moisture, but severe weather is not expected (Figure 5). By Friday, the moisture lessens as a ridge begins to dominate our weather again. We return to the 90’sF and dry out – AGAIN (Figure 6).

Weather Trivia:

The TV stations and weather5280 are throwing out interesting – seasonal – stats. I can’t skip on my trivial duty. DIA has had 36 days of 90F+ weather when it normally has 45 days up to this point. We normally have a 100+F day every other year, and we’ve had two (I believe) at DIA.

Speaking of snow – the average first measurable snow occurs on October 18th. The earliest Denver snow record was Sept 3rd in 1961.

August 14th also saw a new record set for a measured hail stone in our state. It fell near Bethune, CO on August 13th and beat the old record of 4.5″ with 4.83″ long axis and weighted 8.5 oz (some melting occurred before those measurements). I’d want to be underground if that was happening.

Figure 1: The 500mb forecast upper air analysis for Wednesday AM. Pink dot is Longmont. Red lines are troughs, blue lines are ridges.
Figure 2: the water vapor satellite image (browns/reds are dry air, whites and light grey is moist air, purple/blue is ice and high cloud tops). From the the Weather Channel from Tuesday midday.
Figure 3: the water vapor satellite image (browns/reds are dry air, whites and light grey is moist air, purple/blue is ice and high cloud tops). From the the NWS from Tuesday midday.
Figure 4: The severe storm weather forecast for the U.S. from the Storm Predication Center in Norman, OK. Made Tuesday for Wednesday.
Figure 5: The 500mb forecast upper air analysis for Thursday AM. Pink dot is Longmont. Red lines are troughs, blue lines are ridges.
Figure 6: the graphical forecast for the next 10 days for Longmont, CO from weatherunderground.com
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John Ensworth works from Longmont as the Principle Investigator for the NASA Science Mission Directorate Earth and space science education product review through the IGES (The Institute for Global Environmental Strategies – www.strategies.org) . He is in his 14th year running this review. He is an astronomer (from the 2nd grade onward) and became a meteorologist (in the 5th grade) when a thunderstorm in Arizona rained on his telescope when the weather service had only forecasted a 10% chance of rain. He has college degrees in physics and astronomy and climatology and a graduate degree in meteorology and earth science. He lectures at the Little Thompson Observatory in Berthoud, the Estes Park Memorial Observatory in Estes Park, and for a number of online universities. He built and runs a backyard observatory near Pace and 17th in northeast Longmont where he has lived for 8 years with his wife, daughter, son, and two cats. Invitations to open house nights at this observatory, LTO, and EPMO will be posted with future discussions when they are scheduled. Forecasting severe weather and snow amounts via text lead to this column. He began texting friends about the weather right after the September 2013 flood. The readers of this column will, hopefully, keep him honest in what he ‘thought’ he had forecasted for ‘the most recent’ storm.

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