Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 132117
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
317 PM MDT Thu May 13 2021

.SHORT TERM...(This evening through Friday)
Issued at 317 PM MDT Thu May 13 2021

Northwesterly flow aloft will continue through tonight and slowly
back to the west for Friday. At the surface, convergence along a
weak trough/dry line over eastern Colorado may trigger a few
storms. Best chance for this will be this afternoon from Sterling
to Limon and eastward. Surface based CAPE up to 800 J/kg will
support a couple strong to marginally severe storms. The better
chance for organized convection will be to the east over western
Kansas and western Nebraska. For the rest of the night and into
Friday morning expect dry conditions to prevail across the region.
Some cloud cover will hang around and help keep temperatures
warmer tonight with lows in the 40s across northeast Colorado.

A weak wave embedded in the westerly flow aloft will help trigger
scattered afternoon showers and thunderstorms over the higher
terrain Friday afternoon. Surface based CAPE reaches 500-1000
J/kg. This combined with 0-6km Bulk Shear of 35-45 knots will
support strong to severe storms Friday afternoon. The convection
will progress eastward through the afternoon hours. Low clouds
over the northeast corner of Colorado will slow warming and may
keep convection from occurring until the evening. Highs
temperatures will be similar to today with highs reaching the
lower to mid 70s again over northeast Colorado.

.LONG TERM...(Friday night through Thursday)
Issued at 317 PM MDT Thu May 13 2021

Scattered thunderstorms will be ongoing into the evening hours
Friday, possibly organizing into a complex over the northeast
portion of Colorado due to the passage of a shortwave over
Nebraska. There is potential for low level stratus overnight
Friday from convective outflow/front, and a slightly more stable
airmass building into the region. Model soundings favor this
potential with development of a strong inversion near the surface
layer.

Heading into the weekend, the upper level ridge over the region will
flatten as it progresses east while a closed upper level low in
southern California slowly approaches from the west. Flow aloft
will transition to southwesterly while low level flow develops a
more south and east component. This will set the stage for active
weather consisting of scattered showers and storms for each day.

For Saturday, there is sufficient CAPE available along with an
increase in moisture. However, model soundings indicate moderate
capping across most of the plains due to the expected stratus deck
which may limit convective potential. The weakest cap, and thus
the main severe threat looks to develop over the Palmer Divide and
southern foothills. MLCAPE around 1,000 J/kg and 0-6 km bulk
shear in the 30-40 kt range would bring a threat of large hail.

Similar convective potential will exist for Sunday with a broader
area of CAPE around 1,000 J/kg over the region and slightly weaker
0-6 km bulk shear than Saturday. The cap is advertised to weaken,
so more numerous showers and thunderstorms are expected Sunday
afternoon and evening. Precipitable water remains highest over
the eastern plains slightly under 1".

There is sufficient agreement among ensemble guidance with the
closed upper level low slowly progressing east to the south of
Colorado through early next week as an upper level trough pushes
into the Pacific NW. As a result, shower and thunderstorm activity
will continue to persist through the work week; however, majority
of the large-scale forcing remains to the well to the south.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 317 PM MDT Thu May 13 2021

Northerly winds will continue into the early evening with gusts
up to 20 knots. Light winds are then expected for the rest of the
night. The pressure gradient favors weak northerly winds, while
drainage winds could win out as well, so wind direction is
uncertain overnight, however wind speeds will be weak. Low clouds
and patchy fog will likely for over far eastern Colorado. There is
a slight chance this makes it far enough west to affect KDEN.
Threat is low, so will not include it in the TAF at this time.
Scattered afternoon thunderstorms are expected Friday afternoon,
with the best chance being over the higher terrain south of KDEN
and near KAPA. Outflow winds will likely produce wind shifts. If a
strong storm moves overhead, expect brief heavy rain, small hail,
and wind gusts to 40 knots.

&&

.HYDROLOGY...
Issued at 317 PM MDT Thu May 13 2021

Isolated to scattered thunderstorms are expected Friday
afternoon. Brief heavy rain will accompany the stronger storms.
Quicker moving storms and the isolated nature of heavy rain will
keep the threat for flashing flooding low, but can`t be ruled out.

Saturday should see another round of scattered showers and
thunderstorms in similar fashion to Friday afternoon and evening,
again being fast moving with brief heavy rain and a limited flash
flood threat.

Sunday and Monday will likely see more widespread shower and storm
coverage, and potential for heavier rain as moisture levels
increase slightly and storm movement slows down. Snow levels will
remain quite high in the 10,000-11,000 foot range, so most
precipitation falling on burn areas will be in the form of rain.
We may see a slight increase in flash flood threat for the burn
areas both Sunday and Monday afternoon/early evening.

Tuesday could see a continued threat, and we can`t rule out a few
showers and storms through the end of next week due to lingering
moisture and sufficient daytime instability.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$

SHORT TERM...Meier
LONG TERM...Mensch
AVIATION...Meier
HYDROLOGY...Meier/Barjenbruch


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