My Thoughts On May

Since I have been getting into a bit of debate on a weather forum that I frequent, I thought I'd share my ideas for May here.

Coming off of record heat and a scorcher of a month in the East for April, I believe May will have a bit of a different story to tell. With ENSO, NAO, AO and PNA values going into May tracking similarly to those that lead into April, one could think another warm month in the East would be expected. However, given the actual values of the various indices, one would assume the opposite, with warm anomalies in the West and cooler anomalies in the Northeast and Mid-Atlantic. So what happened?

Throughout this month, the Southwest Coast has been receiving a large amount of storms and heavy rain, which is downstream of a ridge that has been in place over the Central Pacific for awhile now. This caused a H-L-H pattern that stretched from the Central Pacific to the Eastern US. The ridge of high pressure across the eastern half of the nation brought in the anomalous warmth that gave 90s to almost the entirety of the Midwest, Mid-Atlantic and Southeast at some point in the beginning of April.

So why will May be different? While the start of May is looking to have a similar pattern to the start of April (with the anomalies being much less extreme in May), models are favoring less cut-off lows just off the California coast than what has happened in April, in order to favor a more northerly track through the Pacific Northwest. This would allow ridging to take a hold in the East Pacific and Western US, which would be the normal conditions for mid to late Spring. Coincidentally, this would also favor a trough in the Eastern US, dominated by storms dropping from the Pacific Northwest into the Central Plains, with a secondary low formation hot-spot in the Northern Gulf of Mexico. This could mean an active period of severe weather in May across the Central and Southern Plains and in the Southeast.

Forecast summary:

Feb. 4-6 Soaker System

NOTE: Forecast is for one storm system, and does not include the impacts of other storms in the US during the same time period.



Another soaker is taking form in the models... which are all in excellent agreement with the timing and positioning of the system at this time. Two pockets of UL energy - one from the Pacific NW and one from northern Mexico - will interact over the South Central US to generate a significant low pressure system. The subtropical portion will draw in plenty of moisture to feed the system, and the Pac. NW portion will provide additional energy behind it, which will help to deepen the low and increase the amount of winter weather associated with this system.

Southern TX will have already received a good portion of rain on Feb. 3, and will receive additional moderate rain as the storm system takes form on Feb. 4. Cooler temps to the north will allow for mixed precipitation and snow in the Great Plains and along the eastern Rockies.

As this system develops and pushes eastward, the vast amount of moisture injected into the system will provide heavy rain across the SE, with mixed precipitation switching over to heavy snow in the Midwest. This swath of precip. will slide east into the Mid Atlantic as the storm moves off the coast.

The severe threat from this system is minimal. Most of the ML and UL air will be drawn in from the tropics, which will weaken the ML and UL lapse rates and hinder convection. Extensive cloud cover will keep surface temps low in most areas, further decreasing the risk of severe weather. However, some low-topped convection could be seen in parts of southern AL and GA and northern FL, which could result in isolated thunderstorms.

Wx Discussion... Jan. 23-25 Central and Eastern US

JANUARY 23-25, 2010
CENTRAL AND EASTERN US SEVERE AND WINTER WEATHER THREAT

A powerful storm is currently that is currently soaking the West Coast will also have a significant impact in the Great Plains eastward to the East Coast. The timing and placement of this storm is pretty consistent between the GFS, ECMWF and CMC model forecasts, which gives me fairly good confidence in this forecast. If the current forecast verifies, we could see severe weather as far north as eastern PA and NJ. This will be dependent upon when the storm occludes, which will be the big determining factor to the northern extent of severe weather.

A strong LL southerly jet will force warm, moist air to migrate northward ahead of a long area of concentrated ML vorticity, which will run north-south over almost the entire N-S length of the US. The UL jet will orient itself SW-NE and provide a great area of lift and will also act to strengthen the center of low pressure as it makes its way from the Central US up through the Great Lakes.

Given the current setup, we could see Slight risks for severe weather extending from the southern US in eastern TX and eastward through most of FL, going north up through parts of MO and extending NE through KY west of the Appalachians. The precip. will redevelop east of the Appalachians and affect most of the east coast up through most of NJ. There will likely be a Slight risk issued for areas east of the Appalachians extending from Central FL up to the DC area, with a See Text from DC up through eastern PA and most of NJ.

Eastern PA and NJ may be in for a strong wind event, should this storm have a favorable development. Most of the ingredients are in place for a severe wind outbreak across this region, which will be created by a well-organized line of storms forming along the cold front near the triple-point where the occluded, cold and warm fronts meet. These events are mostly dynamically-driven, and can be a surprise to many people in this region given the unexpectedness of severe weather during the winter months. Warming at the surface will be the main thing to watch in PA and NJ as this storm develops, as it's temp. relative to the ML and UL temps. will make or break the severe threat in this region.

Now, with all of this exciting severe stuff going on, we cannot discount the winter weather that will be associated with this system. The storm will sit in the Central US for a bit before heading east, which will give it ample time to wrap moisture around the northern and western parts of the center of low pressure. This will cause a wintry mix and a good amount of snow over the northern Great Plains and the western Great Lakes region. The possibility of widespread mixed-precip. over these areas could be disastrous, depending on how far south the rain/snow line reaches during this event. The same areas getting hit by winter weather from the current storm could see additional accumulations of frozen precip. However, areas affected ahead of the center of low pressure may be spared the worst, as rain and wet snow will be prevalent, making travel and cleanup considerably easier.

Here's a map for you all to enjoy:



The 5% (i.e. "See Text") and Slight risks are made using SPC risk standards.

Wx Discussion... Jan. 20-22 Southeast

(This was originally posted on a forum by me on the 18th...)

This is the first of hopefully many weather discussions written by yours truly. To keep myself updating more often, I will not include images, as they take extra time to put on the site, and I do not want to deal with that every thread.

These posts will contain technical discussions, with some words abridged on a consistent basis, such as low/mid/upper level, weather indices, and geographic regions.

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JANUARY 20-22, 2010
SOUTHEAST SOAKER AND SEVERE THREAT

Strong dynamics will come together over eastern TX and LA late on the 20th to produce widespread heavy precipitation over parts of the SE on the 21st and part of the 22nd. A strong LL jet will inject moisture into the SE ahead of a small-but-significant area of ML vorticity. The system's development on the morning of the 21st will be brief, but significant. Strong UL divergence created by the UL jet, along with a negatively-tilted trough axis, will complete the formula for strong dynamics over the SE.

Aiding the chance for severe weather across the SE will be the strong ML and UL lapse rates that will manifest as the dry UL air associated with the Mexican Plateau is advected over the SE. However, the prefrontal thermodynamic instability will be limited by precip. and lingering clouds that will form during the late morning and early afternoon hours, which will weaken the lapse rates out ahead of the area of greatest dynamic lift.

So what does this mean for the SE? Extensive cloudcover will keep severe weather damped, but areas with clearer conditions and more favorable LL and ML lapse rates could see rapidly-developing storms with the potential for damaging hail, strong winds and isolated tornadoes. The 21st will most likely have a Slight Risk for severe weather, extending from MS to FL and northward through TN west of the Appalachians and through SC. The northward extent of the severe weather will be highly dependent upon the strength of the LL jet.

Ample moisture, along with the strong dynamics, will create broad areas of 1-2" of rain across the SE, with localized rainfall approaching 6" in areas that are already struggling with the high rainfall amounts of recent months.

UPDATE:

Added a picture for the severe description: