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Forecast Discussion for Phoenix, AZ

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000
FXUS65 KPSR 102100
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
200 PM MST Sat Apr 10 2021

.SYNOPSIS...
With high pressure slowly being replaced by the influence of a
weather disturbance north of the area, temperatures will steadily
cool back near the seasonal normal by the middle of the week. This
pattern will ensure dry weather continues with locally breezy
conditions during the middle of the week. Temperatures should
gradually moderate somewhat warmer heading into next weekend.

&&

.DISCUSSION...
Early afternoon WV imagery indicates a sharp, high amplitude ridge
has developed west of the region while mean longwave troughing
becomes established over the central Conus upstream of Atlantic
basin blocking. The main weather story through next week will be
hemispheric blocking evolution, retrogressive features, and periods
of persistence type forecasts. Through next week, the SW Conus will
transition from a NW flow pattern into a brief quasi-zonal regime,
then likely into a prolonged period of broadly cyclonic flow. This
will be in response to North Atlantic Rex blocking retrograding
across the entire breadth of Canada to form an impressive East
Pacific/western Conus Omega block. For the forecast area, the
location transition from downstream of high pressure ridging to the
placement in the basal orientation of a Great Basin trough should
ensure a slow, but steady cooling trend while also providing periods
of enhanced winds during the tightest pressure packing.

In the short term through Monday, NW flow will gradually back
towards a zonal configuration as northern stream negative height
anomalies start to retrograde and deepen across the NW Conus. During
this transition, local H5 heights will be eroded from near 576dm to
572dm allowing temperatures to cool a modest 1F-4F. The greatest
uncertainty during this period hinges on timing and thickness of
high clouds poised west of the CWA, and forecast to stream through
the region Sunday afternoon. Have trended somewhat towards the lower
half of the guidance envelope given the prospective cloud cover and
potential negative influence on insolation; though forecast spread
is small to begin with and lower elevation highs in the lower 90s
appear a near certainty regardless of the clouds.

Through the middle of the week, a more pronounced cutoff low
comprising the eastern lobe of the aforementioned Omega block will
settle over the Great Basin. With the forecast area stuck in strong
westerly and broadly cyclonic flow well removed from the nearly
stationary PV anomaly center, height falls will not be overly
aggressive with H5 readings only falling slightly below 570dm.
Nevertheless, this subtle cold advection along with the likelihood
of periods of thicker high clouds should force afternoon high
temperatures back down to near average by mid April standards.
Perhaps the greatest impact from this scenario will be an increase
in winds resulting from the tightening pressure and thermal gradient
across the region. At this time, H8-H7 winds of 25-35kt appear most
probable Tuesday and Wednesday; and efficient spring mechanical
mixing should transport a portion of this momentum to the surface
with the usual areas of terrain enhancement likely.

Not unusual for this type of larger blocking pattern, models are
having tremendous difficulty in determining how the blocking will be
resolved towards the end of next week. Traditionally, blocked
patterns like this change much slower than the majority of models
suggest. And not surprisingly, ensemble spread is rather larger with
a preponderance of GEFS wanting to quickly realign the block while
EPS trends indicate a more stagnant pattern. The preferred and most
likely scenario would keep the cutoff low wobbling somewhere over
the Great Basin while the foreast area stays under dry westerly
cyclonic flow aloft through the end of the week. Eventually, the
pattern is likely to shift again (probably not until next weekend)
resulting in an upward climb in temperatures and high temperatures
rebounding above normal.

&&

.AVIATION...Updated at 1756Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Typical diurnal wind tendencies are favored through the next 24
hours, with periods of variability during diurnal switches. Breezy
conditions are expected this afternoon, mainly between 22-01Z, with
sustained winds up to 10-12 kts and gusts up to 20 kts. Otherwise,
winds will generally remain less than 8 kts. FEW high clouds today
will increase in coverage tonight into tomorrow.

Southeast California/Southwest Arizona including KIPL and KBLH:
Southerly to southwesterly winds expected at both terminals through
most of the day before becoming more westerly this evening.
Sustained speeds will generally be in the 8-10 kt range with
afternoon and evening gusts up to 15-20 kts. High clouds will
increase in coverage through the period.

&&

.FIRE WEATHER...
Tuesday through Saturday:
Temperatures will cool slightly during the middle of the week with
readings falling back to near the seasonal normal. While some
moderation is likely towards the end of the week, readings will
likely remain not too much warmer than average. Afternoon breeziness
will be common with the typical upslope favored areas experiencing
the strongest gusts. Stronger winds will be likely during the middle
of next week yielding a period of elevated fire danger primarily
Tuesday and Wednesday. Dry conditions will also prevail through the
week with afternoon minimum humidity values falling to near 10%
while overnight recovery will be poor only bouncing back to a 25-40%
range. The weather conditions much of next week will provide an
excellent opportunity for prescribed burning operations during the
morning and early afternoon hours.

&&

.SPOTTER INFORMATION STATEMENT...
Spotters should follow standard reporting procedures.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...None.
CA...None.

&&

$$

DISCUSSION...MO/Kuhlman
AVIATION...Benedict
FIRE WEATHER...MO/Kuhlman

Previous Forecast Discussions may be found at
NWS Phoenix, AZ (PSR) Office Forecast Discussions.
(Click 'Previous Version' there to view past versions successively.
Some may differ only in time posted.)

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