Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 062126
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
326 PM MDT Thu May 6 2021

.SHORT TERM...(This evening through Friday)
Issued at 220 PM MDT Thu May 6 2021

We`re finally getting a few little clouds over the foothills but
that should be all we get today. Clear skies and cool air will
make for good cooling, but it will be limited in most places by a
bit of drainage wind so lows should be 5 or 10 degrees warmer than
yesterday.

Substantial warming aloft will be realized at the surface on
Friday afternoon, though there will be some clouds spreading in
from the west. We`ll probably be a few degrees short of the
convective temperature on the plains, but there should be some
weak mountain showers and probably some outflow that may help
produce weak convection on the plains. This is more likely south
of Denver where the capping will be weaker. A dryline will set up
east of Sterling and Akron by late afternoon with a richer
environment on the east side. CAPEs look to be around 1000 J/kg
and hodographs don`t look too bad for a severe threat. This
airmass will likely remain capped, at least in our area during the
afternoon. The marginal risk of a severe storm if there are storms
does look appropriate.

.LONG TERM...(Friday night through Thursday)
Issued at 320 PM MDT Thu May 6 2021

Despite the ridging aloft, there will be a few ongoing showers
and potentially a storm Friday evening over the plains. The best
instability will be across the far northeast corner of Colorado
with mixed-layer CAPE around 1000 j/kg. There may be a storm or
two that taps into this instability and with decent shear, they
could produce large hail and/or strong winds. PoPs were increased
to get a mention of thunderstorms in the forecast.

A cold front will move across the area Saturday morning bringing in
slightly cooler temperatures and better moisture. Aloft, a trough
will move across the area providing QG ascent. It appears the
main axis of this trough will move through during the morning
hours which would not align with peak heating. Showers and storms
are possible in the morning in the mountains and then will form
around midday over the foothills and progress eastward onto the
plains during the afternoon. Areas just to the north of the cold
front will have the best instability and shear Saturday afternoon
and storms that develop there could produce severe hail and
damaging wind gusts. My best guess is that the cold front will be
near the Palmer Divide to southern Lincoln County area by the
afternoon. Therefore, the severe threat should be limited to
Douglas, Elbert, and Lincoln Counties.

A blocking pattern develops by Sunday afternoon over Canada and the
northern US. A trough gets trapped to the south of a large ridge
that will sit over the Saskatchewan providence for a few days. The
exact positioning of each feature will play a big role in the
forecast for early next week. On Sunday, cool, westerly flow aloft
will be over our forecast area creating weak instability. At the
surface, light upslope flow will aid QG ascent with a few showers
and storms developing during the early afternoon. These showers
and storms will spread onto the plains with light rainfall
amounts throughout the afternoon.

By Sunday night, strong QG ascent will develop over Colorado ahead
of an aforementioned trough that begins to sink southward over the
Northern Rockies. Widespread precipitation will develop over our
forecast area as the low level easterly winds increase. This slow-
moving trough will continue to provide lift across our area through
the day on Monday and into Tuesday. Global models and their
ensembles differ on the positioning and speed of this trough. The
ECMWF and Canadian have the trough progressing through the area
faster than the GFS. The GFS is also colder at the surface with
better upslope flow and produces much more snow than the other
models. It appears the GFS is too cold at the surface as the
operational run has a temperature of 33 degrees in downtown Denver
at 3pm Monday. The operational run of the GFS is clearly an
outlier in terms of snowfall in Denver so it was not considered.
Nonetheless, snow is certainly possible in Denver with this
system, especially Monday night, but amounts will likely be light
since temperatures will hover in the mid 30s for the majority of
the event. The forecast was trended toward the ECMWF and Canadian
output with around an inch of precipitation expected in Denver.
This storms should produce moderate snowfall in the mountains and
foothills with highlights eventually possible. If temperatures
trend colder over the next couple days, a threat for a freeze
which could impact flowers/tree is possible over the I-25
corridor.

Ridging will build in over Colorado during the latter half of next
week which will lead to drier and warmer conditions.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 220 PM MDT Thu May 6 2021

VFR through Friday. There is a slight chance of thunderstorms
after 21z Friday. The most likely impact would be a wind shift or
two with gusts up to 30 knots.

&&

.HYDROLOGY...
Issued at 320 PM MDT Thu May 6 2021

There will be a limited threat for flash flooding over the burn
areas on Saturday as storms will be moving somewhat quickly and
won`t be strongest over the burn areas. The threat for flash
flooding over the burn areas will be limited on Sunday as well.
Any storms that develop will be on the weaker side due to low
instability. Moderate to heavy rain will develop over the burn
areas Sunday night and will transition to snow. The threat for
flash flooding will be low Monday and Tuesday due to precipitation
mainly being snow over the burn areas.

&&

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

$$

SHORT TERM...Gimmestad
LONG TERM...Danielson
AVIATION...Gimmestad
HYDROLOGY...Danielson


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