Tropical Disturbance Kicks off the 2024 Monsoon Season

 Overview:

Tropical Strom Alberto is currently in the Gulf of Mexico and is expected to make landfall in Tamaulipas Mexico later today. The disturbance is then expected to track westward across Mexico, potentially emerging over the Pacific Ocean by Saturday or Sunday. Regardless of the exact strength/path of this disturbance, it is expected to bring with it a surge of moisture that will progress from east to west across Texas and New Mexico before arriving in Arizona early Thursday morning. This moisture surge will peak in southern Arizona late Friday and into Saturday as the tropical disturbance moves westward across Mexico. The moisture surge will result in increasing chances of thunderstorm activity starting on Thursday and peaking on Friday or Saturday. Wind profiles on Thursday may support convective organization although moisture will still be somewhat limited. Decent instability, a veering wind profile, and significant potential for avaporative cooling will add to the potential of stong winds in any stroms that do develop on Thursday. After Thursday, wind profiles will be less conducive for convective organization by Friday and Saturday, when moisture availability is at its peak shifting the focus to ther threat of heavy rainfall in slow moving storms. 

Quick Seasonal Outlook:

The seasonal outlooks as provided by the Climate Prediction Center are provided above and indicate that temperatures are likely (50-60% chance) to be above normal while precipitation is leaning (40-50% chance) to the below normal side of the spectrum. It should be noted that these precentages represent a shift from a distribution where each outcome (below normal-near normal-above normal) has a 1 in 3 chance (33% chance) of verfiying. Consequently, a a probability of 40% of drier than normal conditions represents a relatively small shift from a situation whgere each outcome is equally likely. 

This outlook is largely predicated on the expected evolution of sea surface temperatures over the Tropical Pacific as conditions shift from El Nino to La Nina.

Sea surface temperature anomalies as of early June indicated negative anomalies becoming established over the Eastern Equatorial Pacific. This is a prelude to the expected development of La Nina conditions with forecasts indicating a 65% chance of La Nina becoming established by July. This pattern of temperature anomalies in the ocean is typically associated with a late onset Monsoon although the correlations are generally weak.

The Tropical Storm Alberto and Mositure Surge

The latest warning graphic from the National Hurricane Center is depicted below.

While the system is likely to weaken below depression status by Thursday evening, ensemble forecasts indicate a strong likelihood that the remnant circulation will continue westward across Mexico and into the Pacific Ocean

The path of Alberto will be associated with a large plume of precipitable water that forecast by all numerical guidance to move into the region by Thursday morning with precipitable water values in the region exceeding 1" by Thurdsday evening

GFS Precipitable Water Forecast Valid at 5 PM MST June 19th

                                    GFS Precipitable Water Forecast Valid at 5 PM MST June 20th

Once moisture becomes established in the region, it is expected to persist for several days which should support at least the chance for convection for the next several days....

                                    GFS Precipitable Water Forecast Valid at 5 PM MST June 22nd

 GFS Precipitable Water Forecast Valid at 5 PM MST June 24th

Upper Air Pattern:

The figure below shows the 500 hPa heights and winds valid for 5 AM MST this morning:

                            GFS 500 hPa heights/winds  Valid at 5 AM MST June 19th

The important features include a center of high pressure situated over the metropolitan corridor of the Northeast (the much discussed heat dome), the low pressure center associated with Alberto over the southern Golf of Mexico, and the midlatitude trough situated along the West Coast. Over time, the high-pressure system situated over the East will build westward as the trough along the West Coast weakens. This will help steer Alberto westward across Mexico and into the Pacific provided that a remnant circulation survives its trek across the mountains.

                            GFS 500 hPa heights/winds  Valid at 5 PM MST June 20th

By tomorrow evening, the high has extended westward into western New Mexico as the uppe-level low associated with Alberto makes its way towards the west coast of Mexico. 500 hPa winds over Tucson shift to southerly in between the retreating upper-level trough along the West Coast and the Advancing high pressure system. At this point, winds are about as conducive as they will be over the next several days for convective organization. 

                            GFS 500 hPa heights/winds  Valid at 5 PM MST June 22nd

By Saturday evening, the high pressure system dominates the southern tier of the United States from coast to coast. Locally, the center of the high pressure should be situated over central/northern Arizona allowing for weak northeasterly winds over southern Arizona. The weak mid-level flow will mean that convective organization is unlikely after Thurday, but moisture availability will remain high for the next several days. The proximity of the upper-level high isn't exactly favorable for wide spread convection, but the combination of moisture and high temperatures should provide enough instability to ensure isolated to scattered convection for the next several days.

Instability:

Values of Convective Available Potential Energy (CAPE) are forecast to broadly reach values between 500-1000 J/kg in southern Arizona starting tomorrow.

                     HREF Ensemble Mean Most Unstable CAPE Valid for 12 PM MST June 20th

The High Resolustion Rapid Refresh (HRRR) ensemble mean Most Unstable CAPE shows values peaking around noon local time tomorrow with values between 500-750 J/kg. 

                     HRRR UAWRF Sounding for TUS Valid at 12 PM MST June 20th

A closer look at the soundings valid for Tucson from the HRRR initialized UAWRF indicates CAPE values peaking at around 750 J/kg. The veering wind profile with mid-level flow of 20-30 knots in conjunction with Downdraft CAPE values of 1900 J/kg should support a threat for winds approaching severe limits in any convection that develops. Any convective inhibition should be overcome by this time with convective inititation occurring in the UAWRF by about 3 PM local time over the higher terrain just west of Tucson.

                         HRRR UAWRF Simulated Reflectivity Valid at 3 PM MST June 20th

Summary:

An easterly surge of moisture will accompany the passage of Tropical Storm Alberto over Mexico. This will happen at the same time as the high pressure system over the eastern United States currently responsible for the eastern heatwave builds westward. The significant increase in moisture will kick off the monsoon season with a chance of storms for the next several days. However, the proximity of the high pressure system may prevent convective activity from becoming widespread. The primary threat for any storms that develop tomorrow will be strong gusty winds and the potential for dry lightning strikes. As we move into the weekend, there will be increasing chances for storms to be accompanied by heavy rain.