Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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961
FXUS65 KBOU 160344
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
944 PM MDT Sat May 15 2021

.UPDATE...
Issued at 930 PM MDT Sat May 15 2021

There is a bit of convection still going over the northern border
areas of Larimer and Weld Counties at this time. We ended the
watch at 9 PM MDT. Will leave in the mention of fog in the zones.
Will make a few sky cover fixes.

&&

.SHORT TERM...(This evening through Sunday)
Issued at 230 PM MDT Sat May 15 2021

An upper ridge remains over the area with disturbances moving
through it to bring the storm activity to the forecast area.
Easterly flow into the plains keeps dewpoints in the upper 40s to
mid 50s, and PWAT near the max for mid-May. Stratus over most of
the plains is keeping a substantial cap over the plains, shown on
area ACARS data. THe initial storms moving off the foothills have
died quickly in the capped environment. This cap will likely delay
storm activity over most of the rest of the area. However the
Palmer Divide has stayed clear which is where convective
initiation is occurring and the strongest storms are anticipated
due to steep mid-level lapse rates. It will take outflow from the
storms firing over the more unstable mountains to try to break
that cap and allow storms to spread further east onto the plains.
However, the anvils of these storms may delay the breaking of the
cap even further. A Denver convergence vorticity zone, formed by
the turning of the southeasterly winds moving up over the Palmer
Divide, has set up across Douglas County will likely be the focus
of some stronger storms, and possible spin ups with the spin along
the boundary. CAPE values over 1000 J/kg could stretch this spin
into a weak tornado, matching well with SPC`s 2% chance of a
tornado over this area. Otherwise, still expecting the main
hazards this afternoon and evening to be large hail. Areas north
of Denver and across the northeastern plains may not get much
convection at all if this cap and stratus holds on. Decreased PoPs
slightly over this area for that reason. After this initial storm
formation this afternoon, clearing behind it should allow for
better low level warming. Then a disturbance is being realized by
the models to bring a push of more convection from the southwest
near South Park, and pushing across the urban corridor after 6pm.
If enough of the cap has broken by this afternoons activity, this
round could be much stronger with the large hail threat continuing.
This should spread further east out onto the eastern plains this
evening. This second disturbance should have some drying and
subsidence on the backside, with storms dying from west to east.
However there could still be activity over the far eastern plains
after midnight. Outflow combined with the moist easterly flow over
the plains will again bring some stratus overnight.

The upper disturbance will send a northerly push down the urban
corridor early Sunday morning. This may bring fog in the northerly
push for around sunrise and lasting into the mid-morning. Flow
aloft will become slightly more west-southwesterly as a slow
moving upper low digs into Arizona, and the ridge over the state
will stretch back northwest slightly, which will decrease wind
speeds aloft. Upslope flow into the high terrain will be
increasing as a surface low deepens near the Four Corners. CAPE
and shear will be slightly less tomorrow compared to today, so
even less storm organization. However storm motions will be
slower, which will increase the flooding threat over the burn
scars.

.LONG TERM...(Sunday night through Saturday)
Issued at 230 PM MDT Sat May 15 2021

Northern Colorado will remain under an unsettled weather
pattern through the middle of next week with temperatures slightly
below normal and a continuation of likely showers and
thunderstorms. The showers will be most numerous during the
afternoon and evening hours. The culprit will be an upper level
low pressure system which will initially be over West central
Arizona on Sunday evening and then ever so slowly drift/meander
over and across Colorado and surrounding areas on Tuesday and
Wednesday. Long range models are now rather consistent with this
overall pattern and track of the low. There is really only weak QG
ascent with this system but there is ample moisture and a shallow
upslope component to the winds which will remain through mid
week. Precipitable water values will range from .50" in the
mountains to .80" over the urban corridor and around an inch over
the northeast plains. These values will be very close to our
climatological maxes for this time of year. This system will be
warmer than last week`s so any snowfall in the mountains will be
confined to above 10,0000ft.

The Sunday evening timeframe highlights the best chance for heavy
rain with storms in potential for flash flooding (especially in
the burn scars-see more in hydrology section). Nearly saturated
airmass expected with weakening mid level flow and shallow upslope
low level flow. Instability will be more marginal with expected
continued clouds/minimal surface heating so majority of stronger
storms expected foothills and higher plains elevations like the
Palmer Divide. The Monday and Tuesday time frame again will have
ample moisture and near saturation of the airmass but again
instability will be even more marginal for storms. The plains will
have stratus and any surface based CAPE will be over higher
terrain of the foothills and Palmer Divide with values under
800j/kg. For Wednesday, the weakening upper low is right over
Colorado and then shifts east on Wednesday night with slightly
lower chances of showers and some warming.

On Thursday and Friday, the weakening upper low/trof will have
shifted east across the central plains states with temperatures finally
rising to at or slightly above normals for this time of year. However,
a new upper trof will be moving onshore from Northern California
and will increase the southwest flow aloft over Colorado. Despite
a slightly drier airmass, expect more typical rounds of afternoon
and evening showers and thunderstorms each day. Temperatures
across the plains will be in the 70s for highs later in the week.

&&

.AVIATION...(For the 00Z TAFS through 00Z Sunday evening)
Issued at 854 PM MDT Sat May 15 2021

Light winds are expected for DIA all night. Similar to last night.
Cross section hit at some Stratus. Will keep some fog in,
something like 1-3SM BR around 12Z. Ceiling could down into the
BKN010-020 range around 12Z as well.

&&

.HYDROLOGY...
Issued at 230 PM MDT Sat May 15 2021

The heaviest rainfall from this evening`s convection was over
southern Jefferson and the southern half of Douglas Counties.
Radar storm total amounts range from 0.25 to 1.25 inches in those
areas. Significant rainfall amounts did not occur across the CWA
with this evening`s convection. rjk


Some of today`s thunderstorms could produce heavy rains, mainly
south and southeast of Denver where stronger storms are expected.
Rainfall of 1-2 inches in an hour is possible which could cause
minor flooding. For the burn areas in Boulder and Larimer
counties, heavy rain is possible but the threat is lower due to
this morning`s stratus which will keep the airmass cooler and
therefore less unstable. There is a chance of a storm producing a
half inch or rain in less than an hour. West of the Front Range
the threat is lower still, though thunderstorms with brief heavy
rain are possible.

Less instability and organization of storms are expected on
Sunday, however moisture in the atmosphere remains very high,
near the climatological max for mid-May, and winds aloft will be
slowing down to allow for slower moving storms. Model soundings
still show long, skinny CAPE indicating that storms may be more
efficient rain producers than usual. However a lot of cloudiness
is expected starting early Sunday morning which may limit storm
strength. Therefore, will keep an elevated threat for flash
flooding over the burn areas with a marginal threat for flooding
elsewhere.

The best chance for any flash flooding, especially over
the burn scars,will be Sunday evening with rather deep moisture,
weaker storm motions, and a marginally unstable airmass. The flash
flood threat will lower on Monday and Tuesday with even less
instability with more general rainfall and showers with less
intense rainfall from thunderstorms. Slightly drier conditions are
expected from late Wednesday through Friday with more diurnally
driven storms expected.

&&

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

$$

UPDATE.......RJK
SHORT TERM...Kriederman
LONG TERM....Entrekin
AVIATION.....RJK
HYDROLOGY....Kriederman/Entrekin



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