Basement-dwelling pseudo-otaku with a thrill for forecasting on the side.
By: KoritheMan , 2:28 AM GMT on May 07, 2014
A broad area of low pressure is centered several hundred miles south of Manzanillo, Mexico. This disturbance garnered the attention of the National Hurricane Center today, in the form of an off-season Tropical Weather Outlook (TWO) issued at 1:20 PM PDT this afternoon. The actual discussion contained within said outlook I pasted below for reader convenience:
ABPZ20 KNHC 062017
SPECIAL TROPICAL WEATHER OUTLOOK
NWS NATIONAL HURRICANE CENTER MIAMI FL
120 PM PDT TUE MAY 6 2014
FOR THE EASTERN NORTH PACIFIC...EAST OF 140 DEGREES WEST LONGITUDE..
A LARGE AREA OF DISTURBED WEATHER LOCATED SEVERAL HUNDRED MILES
SOUTH OF MANZANILLO MEXICO IS ASSOCIATED WITH AN ELONGATED AREA OF
LOW PRESSURE. THIS SYSTEM IS EXPECTED TO MOVE GENERALLY NORTHWARD
AND THEN NORTHEASTWARD TOWARD THE COAST OF MEXICO OVER THE NEXT
COUPLE OF DAYS. ENVIRONMENTAL CONDITIONS APPEAR TO BE FAVORABLE FOR
SOME DEVELOPMENT DURING THE NEXT DAY OR SO...BUT UPPER-LEVEL WINDS
ARE FORECAST TO BECOME LESS CONDUCIVE WHEN THE DISTURBANCE
APPROACHES THE COAST LATER THIS WEEK. THIS SYSTEM HAS A MEDIUM
CHANCE...50 PERCENT...OF BECOMING A TROPICAL CYCLONE DURING THE NEXT
48 HOURS...AND A MEDIUM CHANCE...50 PERCENT...OF BECOMING A TROPICAL
CYCLONE DURING THE NEXT 5 DAYS. REGARDLESS OF DEVELOPMENT...LOCALLY
HEAVY RAINS COULD AFFECT PORTIONS OF SOUTHWESTERN MEXICO OVER THE
NEXT FEW DAYS.
ANOTHER SPECIAL TROPICAL WEATHER OUTLOOK WILL BE ISSUED BY 8 AM PDT
WEDNESDAY...OR SOONER IF NECESSARY.
Following the low cloud lines via visible satellite imagery from 18z to 0z, I am convinced that the increasingly better-defined center is located more or less in the center of the convective mass, an assessment that merits additional weight coincident with extrapolation of a timely 2343Z SSMI microwave pass that was fortunate enough to just capture the center. While there are no signs of convective banding yet, there is some evidence that the outer convection of the circulation is attempting to cyclonically consolidate toward the center of the surface low. Upper-level outflow is radiating sprightly to the north and northwest of the system for several hundred miles.
Figure 1. Latest infrared satellite image of Invest 90E. Image credit: NOAA's Satellite Services Division (SSD).
There is some evidence that an upper-level anticyclone is beginning to become collocated atop the surface low, an environment conducive for additional organization. I've dropped an image below showing the difference in the upper wind pattern aloft comprising the period 1800Z to 0000Z. Both images are creations of the University of Wisconsin CIMSS group.
Figure 2. Homogeneous comparison of the upper-level wind flow around Invest 90E from 1800Z May 5 to 0000Z May 6. The outflow level is typically found around 200 mb, so the light blue barbs are the ones we care about the most. Notice the fanning out of those same wind barbs toward the north and northwest, indicating a lessening of the upper-level shear as the upper-level trough responsible for it over the last couple of days fills in advance of another trough approaching from California.
The SHIPS, which uses the GFS formula for its own forecasts, suggests an environment of warm sea surface temperatures and low vertical shear over the next day or two, with the possibility of shear increasing in about 24 hours. Looking at water vapor imagery, the shear downstream of the disturbance does not appear to be as inhibiting as it has been over the last couple of days, so it's possible that the shear could hold off a little longer than the SHIPS/GFS are suggesting. Even if the shear does increase (and it should), the GFS shows it increasing from the south, which would not be as detrimental to a northward-moving disturbance as shear over 20 knots would ordinarily be.
All of the global models forecast the disturbance to not intensify beyond a marginal tropical storm, and there is even some suggestion amongst them that the system could actually start to weaken prior to landfall. While increasing shear blowing in the same direction as the storm movement is not in and of itself sufficient to incapacitate the disturbance, recall that the mountains of southern Mexico are excessively large, and when considering land interaction along with the concurrent increase in shear, the global model projections of some weakening prior to landfall is not out of the realm of realistic possibilities. Nevertheless, most of the models do forecast some short-term intensification, and even the ECMWF, while being its usual not-showing-a-tropical cyclone-self, has gotten a little more aggressive today in actually showing a broad low pressure area with the disturbance, which I didn't notice in the respective model fields yesterday.
Given this and current convective trends -- particularly the fact that the western low has all but dissipated as correctly forecast by the GFS -- I anticipate a tropical storm forming tomorrow or tomorrow night. I wouldn't be too enthusiastic about intensification potential after that, maybe up to 45 kt at the very most, with the likelihood of some weakening as the system approaches the coast on Thursday.
The system is currently wedged between an upper-level trough to the west and an upper-level ridge to the east, which is causing the system to move on an east-of-north course, toward the southern coast of Mexico. Global model guidance and UW-CIMSS steering analysis suggests this general motion should continue up until landfall. There is some difference in the models, with the ECMWF and NAVGEM showing a more prolonged northward movement which ends up being a little to the west of the landfall points for the GFS and GEM. I am inclined to blend the guidance, with a bit of a bias toward the GFS/GEM.
Regardless of development, heavy rainfall, gusty winds, and flash flooding in areas of enhanced terrain will occur by as early as tomorrow.
Probability of development in 48 hours: 60%
Probability of development in 120 hours: 60%
The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.
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