Calexico, CA 92249 Weather

Update

Updated Aviation Discussion.

Key messages

- Tranquil weather conditions will continue today and Thursday with temperatures within a few degrees of daily normals.

- An unsettled weather pattern will return to the region from Friday through Sunday resulting in much cooler temperatures and enhanced chances for widespread rainfall.

- Embedded thunderstorms and heavier rainfall may affect some areas resulting in localized minor flooding, specifically Friday and Sunday.

.SHORT TERM /Today through Thursday/: The current quiet weather conditions will abruptly come to an end by Friday morning as the first of two weather systems affects the region. Until then, other than some increasing higher level clouds, not much will change. Our current westerly dry flow will slowly shift more out of the southwest later today as a large Pacific trough takes shape off the West Coast. The split flow pattern supporting a strong upper low along the British Columbia coast and a second much weaker disturbance well west of California will stay distinct over the next couple of days, but that will change by the weekend. In the near term, the air mass currently in place will hold fast, but with some upper level moisture shifting in from the northwest bringing some fairly thin high clouds today. A second batch of high level moisture and clouds will move in directly from the west on Thursday, while increasing southerly flow in the low levels will also begin to advect subtropical low level moisture northward into south-central Arizona. Despite the increased cloudiness today and Thursday, daytime highs should hold right near averages for late October.

.LONG TERM /Friday through Tuesday/: Forecast confidence is increasing for two separate weather events, Friday through Sunday, both of which could bring impactful rain amounts to portions of central Arizona. The increasing moisture on Thursday into Friday is expected to peak during the daytime hours Friday with PWATs reaching 225-250% of normals and low level mixing ratios of 8-9 g/kg, centered over Maricopa and Pinal Counties. Forecast soundings show the moisture will be quite deep, extending fairly evenly throughout the column with modest elevated MUCAPE up to around 500 J/kg. Strong jet forced ascent is also seen moving over the area on Friday combining with the modest southerly low level upslope ascent.

Showers are expected to begin developing a few hours after midnight Thursday night before increasing in coverage and intensity by around sunrise Friday. Both the GEFS and EPS show QPF maxima centered from Phoenix to around the Tonto NF with amounts averaging 0.50-0.75". Considering the available instability and steep mid-level lapse rates, we should expect scattered thunderstorms too, mostly embedded with the shower activity. General rainfall rates are likely to mostly fall within a 0.05-0.2" range, but it would not be surprising to see very localized rates of around 1" per hour within any thunderstorms. These higher rainfall rates should pose a minor flood threat on Friday, mainly affecting area washes and the typical unbridged low water crossings. There is potential for higher rainfall amounts with the NBM 75th percentile showing amounts between 1.5-2.0" centered over the foothills just north of Phoenix. A Slight Risk for excessive rainfall remains on the updated WPC ERO and based on the model run-to-run consistency, Flood Watches are likely to be needed for at least north-central and northeast Maricopa County. The shower and thunderstorm activity should also extend eastward into Gila County later on Friday before quickly winding down Friday evening as we lose upper level support.

The large scale trough will remain positioned along and into the Western U.S. into Saturday with the main low center near the Pacific Northwest, but another strong shortwave diving into the base of the trough is expected to help bring another low developing off the coast of California Saturday night into Sunday. Saturday is expected to be mostly a dry day, but there will still likely be some isolated upslope type showers mostly focused over the high terrain north and east of Phoenix. However, this will just be the calm in between weather systems with the second one moving in starting late Saturday night expected to be stronger and more impactful.

By Saturday evening, guidance shows a strong atmospheric river (AR) starting to make its way into northern Baja and far southern California. By Sunday morning, the full force of this AR is expected to be directed at southern Arizona and northern Sonora Mexico with PWATs increasing to 275-325% of normal, or right near record levels for our area this time of year. Compared to model runs 24 hours ago, the latest depicts even better moisture and a stronger system. Upper level forced ascent should begin to increase over our area by Saturday night, coinciding with increasing low and mid level upslope south southwesterly flow.

A large area of light to moderate rain is likely to develop ahead of the approaching weather system late Saturday night into early Sunday affecting the bulk of the region, particularly central and eastern Arizona. Although model uncertainty is decreasing, there is still a good deal of uncertainty in expected rainfall amounts. This looks to be a longer duration light to moderate rain event with limited instability keeping rainfall rates under 0.5" per hour and even lower across the lower deserts. However, because this may end up easily being a 6-12 hour rain event, the rainfall could quickly add up, especially into the higher terrain northeast of Phoenix where upslope forcing will be maximized. The latest NBM QPF shows a wide range of potential amounts with averages around 1" in the Phoenix area to upwards of 1.5-2" over the high terrain northeast of Phoenix. The lower end of the forecast shows amounts closer to 0.5" and 1-1.5" respectively, but the higher end potential is more like 2-3". Considering this will be the second rain event within 3 days, any additional moderate to heavy rain could compound any flooding issues that occur Friday. Rain chances may linger through Sunday night across portions of central and eastern Arizona, but by mid to late Monday morning, much of the area should be drying out as the system is quickly pushes east of the area.

Temperatures during this unsettled weather pattern will fall below normal starting Friday with highs dropping to between 75F-80F before bottoming out closer to 70F highs for Sunday and/or Monday. It would not be out of the question for some lower desert communities stuck in prolonged rain Sunday to not even breach the 70F threshold. NBM guidance shows fairly large forecast temperature spread for the first half of next week as the large scale troughing may linger near the region with depressed H5 heights for a couple more days. If this happens, highs will very slowly creep upward through the low to mid 70s, whereas if the ridge quickly takes over, highs closer to 80F would be more common by next Tuesday or Wednesday.

Aviation

Updated at 1100Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT; and Southeast California/Southwest Arizona including KIPL and KBLH:

No major aviation concerns will exist through Thursday morning under periods of thicker high cirrus. Winds will remain light, generally AOB 8 kts, with a tendency towards SE'rly to S'rly directions during the daytime hours today. Confidence is low on a full westerly wind shift at the Phoenix area terminals, though a light W/NW component could develop for a few hours around sunset. Otherwise, expect extended periods of variability to nearly calm conditions.

Fire weather

Dry conditions with near normal temperatures will persist through Thursday under increasing cloud cover. Minimum humidity levels will fall into a 20-40% range following fair to good overnight recovery of 40-60%. Winds will remain unseasonably light with minimal upslope breeziness. Beginning Friday, several days of unsettled weather will affect the region with much higher humidities, widespread rainfall, and cooler temperatures.

Watches, warnings and advisories

AZ: None. CA: None.