613
WTNT41 KNHC 100251
TCDAT1

Subtropical Storm Karen Discussion Number 1
NWS National Hurricane Center Miami FL AL112025
300 AM GMT Fri Oct 10 2025

Over the last 12-18 hours, NHC has been monitoring a small area of
low pressure (AL96) embedded in a much larger upper-level trough,
located about 400 n mi to the northwest of the northernmost Azores
Islands. The system was originally a frontal low that quickly became
occluded early on 8 October. Since that time, the low has been
gradually shedding its remaining frontal features as shallow to
moderate convection formed near the center. Earlier today, there was
a Sentinel-1A Synthetic Aperture Radar (SAR) pass that indicated the
system no longer was attached to frontal boundaries and had a
contracted radius of maximum wind of 20 n mi. Since that time, the
convective cloud tops have cooled to -50 to -55 C around the center,
with a distinct warm spot seen near the center. A recent WSFM
microwave pass also indicated a nearly closed cyan ring on the
37-GHz channel. 00 UTC subjective satellite classifications on the
system were a ST1.5/25-30 kt from TAFB, and a T3.0/45 kt from SAB. A
blend of all of these various data points indicate that the system
has made the transition to Subtropical Storm Karen. The initial
intensity is set at 40 kt, in agreement with a recently received
scatterometer pass which had a peak wind of 38 kt. This also matches
the special 02 UTC fix of ST 2.5/35-40 kt received from TAFB.

The subtropical storm is moving slowly northeastward, estimated at
050/8 kt. Karen is already well established in the mid-latitude
westerlies for being at such a high latitude, and is likely to be
caught up by another mid-latitude trough that is digging in from the
west. The track guidance in in pretty good agreement on a
northeastward motion with a gradual acceleration over the next 24-36
hours, and the NHC track forecast is roughly in the middle of the
guidance envelope, close to the simple and corrected consensus
aids.

While Karen has made the transition to a subtropical storm, it still
remains fully embedded in a large upper-level low, so large in fact
that it has helped the shear over the system remain fairly low and
has enabled the convection to coalesce near the center. Very cold
upper-level temperatures also appear to be contributing to the
convective vigor, allowing cloud tops of -50 C to persist near the
center. However, even colder sea-surface temperatures lie ahead of
Karen's path, and it seems like the system only has about a day or
so before it is quickly overtaken by the next mid-latitude trough.
The global models show the small cyclone opening up into a trough by
48 h, and it is also possible the system could become post-tropical
before then if it loses its current organized convection. This
intensity forecast is in general agreement with the mean of the
intensity guidance.


FORECAST POSITIONS AND MAX WINDS

INIT 10/0300Z 44.5N 33.0W 40 KT 45 MPH
12H 10/1200Z 45.6N 31.7W 40 KT 45 MPH
24H 11/0000Z 47.8N 29.5W 40 KT 45 MPH
36H 11/1200Z 50.6N 28.0W 35 KT 40 MPH
48H 12/0000Z...DISSIPATED

$$
Forecaster Papin


= = = = = = =
To unsubscribe from WX-ATLAN and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at [email protected] and ask to be removed from WX-ATLAN.

--- SBBSecho 3.28-Linux
* Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105)