Sat Jun 01 2019 12:07 AM
Invest 91L Lounge

A broad area of low pressure is percolating in a seasonably favorable region for tropical cyclone genesis, and this feature has now been tagged as Invest 91L, our second Invest of the 2019 Atlantic Hurricane Season.

NHC saw it appropriate to issue a preseason TWO on this disturbance earlier today:

Special Tropical Weather Outlook
NWS National Hurricane Center Miami FL
220 PM EDT Fri May 31 2019

For the North Atlantic...Caribbean Sea and the Gulf of Mexico:

1. A broad area of low pressure accompanied by cloudiness and showers
centered over the Yucatan Peninsula is forecast to move westward
over the southern Bay of Campeche during the weekend. Some gradual
development of this system is possible through early next week as
long as it remains over water. Regardless of development, the
disturbance will likely produce heavy rainfall over portions of
southern Mexico during the next few days. Regular issuance of the
Tropical Weather Outlook will begin at 2 AM EDT tonight with the
beginning of the Atlantic hurricane season.
* Formation chance through 48 hours...low...20 percent.
* Formation chance through 5 days...low...30 percent.

Forecaster Avila/Zelinsky

Models have been warming up to cooking this one up a bit, and, provided it can remain mostly or all over water, there is also a real possibility of 91L becoming a TC or TC-like system that dumps heavy rains over Mexico and/or recruves into one or more Gulf Coast states sometime next week. As such, we are now opening up a "Forecast Lounge" on this disturbance.

The title will be updated if and as warranted.

Sat Jun 01 2019 04:05 PM
Re: Invest 91L Lounge

NHC increased development odds to 60% with the 2PM TWO and this could be conservative. In fact, it is now debatable whether or not 91L is already classifiable, and it would not surprise to see Advisories begin sometime late tonight - at least on a "PTC," or "Potential Tropical Cyclone" basis, assuming it hasn't already been upgraded. This necessitates TC-centric model runs (HWRF, HMON, etc.), and we should begin to see those start coming in shortly.

Considering that 91L looks to be getting its act together a bit ahead of schedule and a bit more robustly, additional attention should be paid to the more aggressive model runs we have seen thus far. Of these, the often accurate ECMWF does crank 91L up into what would likely be Barry, and potentially a high-end Tropical Storm Barry at that. Movement per this model's most recent run would be gradually and unevenly into Tampico, MX with an eventual recurvature along or just offshore of the Texas coastline before ultimately making a final landfall (of possibly several along its route - Mx/Tx/La) on Louisiana around Friday of next week.

Sun Jun 02 2019 01:45 AM
Re: Invest 91L Lounge

Much of the low to mid level circulation associated with 91L already appears to be over land, and this may be why the intensification trend seen during the day Saturday seems to have halted, with convection even waning into the overnight hours. With a general west to west-northwest trajectory in the near term, it is starting to look increasingly possible but not yet probable that 91L just doesn't pull itself together before it is too late, and such an outcome is now being represented by the bulk of the primary models.

Recon has been tasked to investigate 91L later today, Sunday, if necessary. If by sunrise 91L doesn't look any better than at present, especially if even more of its circulation has entered land, today's flight could be canceled.

Recently a renewed burst of convection has fired up in what could generously be called the northeast quadrant, with what looks like some effort to wrap and build. Time is of the essence.

Regardless of development, 91L, whatever becomes of it, will likely interact with a mid-latitude low now approaching from its northwest to produce copious, multi-day rains over eastern Mexico and possibly the eastern third of Texas as well. The atmosphere over south Texas is already seeing PWAT values of about 1.5" and modeling suggests a fetch of 2.0" or even better overspreading the region Tuesday into Wednesday. Combined with the aforementioned mid-lat system, scattered to widespread showers and storms, some possibly locally heavy and/or severe, would be a conceivable outcome.

Sun Jun 02 2019 03:56 PM
Re: Invest 91L Lounge

Some 6/2 12z Model overviews

ECMWF - Nowhere near as aggressive as 12z yesterday, now only working 91L into a *maybe* TD just south of Brownsville, TX on or about Tuesday/Wednesday. Still a prolific rain-maker, widespread moderate to heavy showers and perhaps some thunderstorms implicated over much of south Texas into Louisiana as 91L, or by then perhaps TD2, is drawn into the upper-level trof approaching from its northwest.

GFS - Similar to the EURO above, but with even less of a TD signal.. most likely drawing 91L up as an inverted trof into the upper-level trof from its northwest, along with very high PWATS into deep south Tx, east Tx, Louisiana, and later in the week much of the south.

CMC - Has a much stronger TD signal than today's 12z ECMWF and GFS. Draws 91L or TD 2 into deep south Texas and rides the center of lowest surface pressure right up and along the Texas coast from near Brownsville, Tx Wednesday and pushing inland into Louisiana by Thursday night. Like the other two models mentioned, a stout feed of high PWATS is drawn into deep south Tx, east Tx, Louisiana and eventually many other southern states.

Generally, today's 12z runs of the major global models offer up a roughly 50/50 or so chance that 91L becomes a TD before moving inland over or near south Texas and maybe Louisiana, with the greatest threat likely to be ongoing showers and tropical downpours, with an attendant flood risk.

All of these runs rely on the 'center' of 91L remaining much closer to extreme southwest Bay of Campeche, and may be less helpful if the trend of 91L reestablishing itself further offshore continues.

Mon Jun 03 2019 03:56 AM
Re: Invest 91L Lounge

91L's structure gradually improved heading into the evening Sunday, and this trend has continued at a somewhat faster pace so far overnight. The low now no longer appears disorganized, but rather looks much more like a wanna-be TD again.

There are two main considerations as to 91L's future forecast track worth mentioning.

First, the exact 'center' of 91L is still a bit of a mystery. This is often the case with developing tropical systems, especially when they are not yet even in the TD state, as 91L has been. The best, albeit elongated, surface circulation remains a good bit onshore in the southwesternmost portion of the Bay of Campeche. However, the circulation just above the surface appears to have reconfigured itself further offshore during the past 24 hours, and this is also the location now officially being tracked:

We likely won't have a really good handle on where the exact center is until recon arrives on the scene later today, Monday. This initial position of the low could have outsized impacts on the accuracy of model forecast tracks and intensities down the road, so we'll want to start off with getting that right.

For now, assuming that the estimated center location is 'close enough' to correct, recent model runs shown below as plotted by SFWMD for 02/12, 02/18z and 03/00z strongly suggest a consensus that the low could stay mostly if not entirely offshore until a final or eventual landfall around Louisiana mid to late week. However, the track range, or de facto "Cone of Uncertainty," does cover hundreds of miles.

I've outlined in heavy black pen the range, with the individual members (e.g. GFS, HWRF, UKMET) within. Note: XTRP is extrapolating of the past several hours of actual track - but in this case it is not really valid as such, because rather than a traditional track towards the northeast, we have merely witnessed a center jump. So unless that trend were to continue, a movement more towards the northwest or northish should resume in the near term.

Wed Jun 05 2019 05:41 PM
Re: Invest 91L Lounge

Really easy to see how 91L is acting like a sheared TD today. Not only acting like, but in many ways, looking like.

Invest 91L Visible w overlays June 5, 2019 2131z
Credit: College of DuPage

Wed Jun 05 2019 05:49 PM
Re: Invest 91L Lounge

WPC Day 1 Excessive Rainfall Discussion June 5, 2019 307PM


Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
307 PM EDT Wed Jun 05 2019

Day 1
Valid 1902Z Wed Jun 05 2019 - 12Z Thu Jun 06 2019



...Eastern Texas into Southwest Louisiana...
1900 UTC Update...Pared the western peripheries of the outlook
areas (across the western Gulf Coast) based on the latest
observational/mesoanalysis trends along and west of the MCV and
associated surface low track. -Hurley

1600 UTC Update...Made minor modifications to the High and
Moderate Risk areas based on the current radar and mesoanalysis
trends, the latest FFG/soil moisture analysis, and 12Z CAM
guidance. A more notable adjustment was the eastward expansion of
the Slight Risk area through LA, more of southern-central MS, and
into southern AL. Observed PW values over 2.5" (including 2.54"
per the 12Z CRP RAOB) are well above the climatological norms, and
very close to June records. The degree and depth of the
highly-favorable thermodynamic environment (PWs 2.25-2.5+ inches,
wet bulb zero levels above 15kft, and tall/skinny CAPEs near 1000
j/kg) will maintain optimal warm rain processes and
highly-efficient rainfall rates (2+ inches in an hour). -Hurley

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