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Area Forecast Discussion (AFD)

Issued by Denver/Boulder, CO (BOU)

FXUS65 KBOU 171528

Area Forecast Discussion
National Weather Service Denver/Boulder CO
928 AM MDT Mon Jun 17 2019

Issued at 917 AM MDT Mon Jun 17 2019

Main concern today is the threat of severe thunderstorms across 
the northeast plains of Colorado this afternoon and evening. Water
vapor imagery showing several weak waves which will provide some 
weak synoptic scale lift over the entire area through tonight. 
There are already showers that have been traversing our northern 
borders and another pocket of showers across the southern Front 
Range foothills this morning. So far, all of these are rather 
light showers.

Moisture levels are higher today, as precipitable water values
around .80" of an inch based on GPS. Quite a bit of cloud cover 
this morning over the Front Range with dissipating stratus deck 
and a batch of middle level clouds above the stratus. There is 
more initial sunshine farther east over the plains which will 
likely help in achieving greater instability there. Further west 
over the Front Range, the cloud cover will likely keep the 
airmass at least initially more stable through early afternoon. 
No 12z DNR sounding but new NAM forecast soundings show stable 
layer will persist until around 20z, which could be a bit 
optimistic based on current cloud cover. Initial storm coverage 
coming off the mountains and large scale lift will likely help to 
convection to get going by mid afternoon. Forecast CAPE values 
generally 1000-2000j/kg, again highest east of the I-25 corridor 
with the best threat for severe storms. Impacts will be large hail
and heavy rain given the high PW values seen this morning. Since
storm motions will be 10-15kt don't expect a huge flash flood
threat today but certainly advisory levels for water may be
needed. Expect several waves of showers and storms through at 
least this evening given the disturbances aloft.


.SHORT TERM...(Today and tonight)
Issued at 442 AM MDT Mon Jun 17 2019

There will be a slight risk of severe thunderstorms this afternoon
east of a line from around Greeley to Byers to Monument this 
afternoon, with a marginal risk along and west of the I25 
Corridor. The strongest storms along the I-25 corridor are 
expected to occur between noon and 3 pm, then the focus will shift
eastward across the northeast plains. The latest HRRR/RAP13 show 
a secondary wave of weaker thunderstorms develop over the Front 
Range this evening, with some lingering showers overnight. 
Forecast CAPES this afternoon around 2000 j/kg for Denver, 
2500-2900 j/kg over the northeast plains. Precipitable water 
values 1-1.1 inches as well, so locally heavy rainfall may be 
concern as well, Main issue will be large hail up to 2 inches and 
damaging wind gusts. Best wind shear profile will be around 21z. 
Storm motions around 10 kts, so leaning more towards advisory 
criteria rainfall vs flash flooding at this time. 

Models show generally weak qg ascent in the mid levels this afternoon
with a mid level trough axis brushing across northern Colorado 
around 00z. At the surface, low pressure will form over southeast 
CO by midday, allowing for a persistent east to southeast winds 
through early this afternoon, with stronger northerly winds spreading
southward into the cwa late this afternoon and evening. Isolated
thunderstorms over northeast Larimer and over Sedgwick and
Phillips counties at this time, should diminish by 12z. Patchy
fog also expected north and northeast of Denver until around 14z. 

.LONG TERM...(Tuesday through Sunday)
Issued at 442 AM MDT Mon Jun 17 2019

Pretty good model agreement through this week. On Tuesday showers
associated with Monday's little low will be moving out of the 
northeast corner in the morning while moisture ahead of the next 
trough will be moving into the mountains. There should be a break 
between these two systems though, so we lowered PoPs for much of 
the day across the plains, with the expectation of showers/storms 
spreading across the area from west to east during the afternoon 
and early evening. Guidance is trending cooler for Tuesday and 
depending on the clouds and showers it may stay in the 60s in the 
northeast corner. This area may cool enough to be too stable for 
the second round of storms to make it that far east. 

Drier air will move in behind the Tuesday wave and it looks like
there will be enough flow for some downslope winds east of the
Front Range later Tuesday night into Wednesday. Not particularly
strong, but it will help to dry out the lower levels. There should
also be significant warming, so Wednesday looks like a day with a
pretty low level of convection and highs in the lower to mid 80s
on the plains. Thursday looks like a similar environment, except
some moisture return over the eastern part of the plains and a jet
streak could create some storms, and possibly even some marginally
severe storms near the eastern border as the wind profile will be
pretty good. There may also be a little small-scale front 
associated with the jet streak. We'll see if the moisture return 
is enough yet or if that will have to wait for Friday.

It looks like there should be an uptick in convection starting
Friday as a trough develops over the west. There is fair agreement
on a lead shortwave trough that would give some lift, and enough
flow for lee cyclogenesis around southeast Colorado that will draw
moisture westward. This may be a severe weather day with the 
moisture more than making up for a little cooling and much better
shear. For the weekend, there should be continued west to
southwest flow aloft depending on the shortwave details, but there
will likely be some cooling, a still moist airmass, and some lift
at times to provide a chance of thunderstorms. The blended model
solution looks good for this.


.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 917 AM MDT Mon Jun 17 2019

Expect stratus deck to slowly dissipate between 16-17z over 
terminals but given the mid level deck of clouds over the area
wouldn't be surprised that it waits to 18z at KDEN. Expect several
round of showers/thunderstorms today, with first round in the 20- 
21z time frame. Heavy rain and hail may result in brief ILS
approaches during the storms. Could see some lingering rain
showers and low clouds again later tonight following the




LONG TERM...Gimmestad