Advisories

WDPN33 PGTW 260900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 20W (TWENTY) 
WARNING NR 001//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 17.2N 161.3E
   INITIAL INTENSITY: 25 KTS
   GEOGRAPHIC REFERENCE: 327 NM WEST-SOUTHWEST OF WAKE ISLAND
   MOVEMENT PAST 6 HOURS: NORTH-NORTHWESTWARD AT 04 KTS
   SIGNIFICANT WAVE HEIGHT: 10 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS
PERSISTENT CONVECTION OVER A VERY COMPACT LOW-LEVEL CIRCULATION
CENTER (LLCC), FLANKED BY A WEAKLY CURVED BANDING ON THE NORTHWEST
PERIPHERY. A 260519Z SSMIS 37 GHZ IMAGE REVEALED THE EXISTENCE OF A
NASCENT LOWER-LEVEL CORE STRUCTURE. THE SYSTEM IS VERY SMALL, SO
THE MICROWAVE IMAGERS AND SCATTEROMETERS HAVE DIFFICULTY IN
RESOLVING THE STORM FEATURES. A 252149Z ASCAT-B IMAGE ESTIMATED
NON-FLAGGED WINDS OF 20 KTS, AND GIVEN THE SATELLITE PRESENTATION
IN EARLY VISIBLE SATELLITE IMAGERY, IT IS POSSIBLE THAT THESE
VALUES MAY BE LOW. THE ENVIRONMENT IS CONDUCIVE FOR FURTHER
DEVELOPMENT, CHARACTERIZED BY LOW VERTICAL WIND SHEAR (VWS), RICH
DEEP-LAYER MOISTURE, AND VERY WARM SEA SURFACE TEMPERATURES (SST).
THE INITIAL POSITION IS ASSESSED WITH HIGH CONFIDENCE BASED ON
PARTIAL CLEARING OF CLOUDS NEAR THE CENTER AND THE TIGHT GROUPING
OF AGENCY FIXES. THE INITIAL INTENSITY IS SET AT 25 KTS BASED ON
THE SUBJECTIVE DVORAK FIXES OF T1.5 FROM PGTW, RJTD, AND PHFO. THIS
ESTIMATE MAY BE CONSERVATIVE GIVEN THE SYSTEM'S SIZE AND THE T2.0
FROM KNES.

INITIAL WIND RADII BASIS: NOT APPLICABLE (THERE ARE NO INITIAL WIND
RADII).

CURRENT STEERING MECHANISM: NORTHWESTERN FLANK OF A LOW- TO
MID-LEVEL NEAR-EQUATORIAL RIDGE (NER)

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T1.5 - 25 KTS
   RJTD: T1.5 - 25 KTS
   KNES: T2.0 - 30 KTS
   PHFO: T1.5 - 25 KTS
   CIMSS D-MINT: 30 KTS AT 251807Z
   CIMSS D-PRINT: 30 KTS AT 260600Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 10-15 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE POLEWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: MEDIUM
   INITIAL WIND RADII: NOT APPLICABLE

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THIS INITIAL PROGNOSTIC REASONING
MESSAGE ESTABLISHES THE FORECAST PHILOSOPHY.

FORECAST DISCUSSION: AFTER BEING ON A NORTH-NORTHWESTWARD
TRAJECTORY, TROPICAL DEPRESSION (TD) 20W IS TURNING TOWARD THE
EAST. THE SYSTEM WILL TRAVEL NORTHEASTWARD THROUGH TAU 36 ALONG THE
NORTHWEST FLANK OF A NER POSITION. THIS RIDGE BECOMES MORE ORIENTED
NORTH-TO-SOUTH AS THE SYSTEM GAINS LATITUDE, WHICH WILL ENABLE A
POLEWARD TURN AND INCREASE IN FORWARD SPEED AFTER TAU 36. WHILE THE
TRACK PHILOSOPHY APPEARS STRAIGHTFORWARD, SIGNIFICANT UNCERTAINTIES
EXIST. A BROADER CYCLONIC CIRCULATION, DESIGNATED AS 91W AND
CURRENTLY LOCATED SOUTHEAST OF WAKE ISLAND, MAY INTERACT WITH TD
20W DURING THE NEXT 48 HOURS. THERE IS A DISTINCT POSSIBILITY THAT
91W WILL INTENSIFY OVER THE NEXT FEW DAYS AND PULL TD 20W INTO THE
LARGER CIRCULATION, EFFECTIVELY CAUSING THE TD TO DISSIPATE. FOR
NOW, THE JTWC FORECAST KEEPS TD 20W INTACT AND DISTINCT FROM 91W.
THROUGH TAU 60-72, THE VWS WILL BE LIGHT, AND THE SST WILL BE HIGH.
THEREFORE, THE FORECAST CALLS FOR SLOW DEVELOPMENT AT FIRST AS TD
20W BECOMES BETTER ORGANIZED. ONCE IT ESTABLISHES A MORE DEFINED
INNER CORE, IT WILL INTENSIFY MORE QUICKLY. BY TAU 60-72, RAPIDLY
DECREASING OCEAN HEAT CONTENT WILL PUT A LID ON THE
INTENSIFICATION. WEAKENING WILL COMMENCE BEYOND TAU 72 AS THE
SYSTEM ENCOUNTERS COOLER SST AND HIGHER SHEAR.

MODEL DISCUSSION: THE MAJORITY OF THE MODEL TRACKERS ARE VERY TIGHT
WITH A CROSS-TRACK SPREAD OF 60 NM AT TAU 72. HOWEVER, NAVGEM AND
GFS HAVE A DISTINCT SEPARATION FROM THE CORE ENVELOPE AS THE
EASTERN OUTLIERS. THESE MODELS PREDICT THAT 91W WILL ABSORB TD 20W.
THE JTWC FORECAST TRACK ALIGNS WITH THE CORE GUIDANCE ENVELOPE AND
FOLLOWS MOST CLOSELY THE MULTI-MODEL CONSENSUS (CONW). THE JTWC
INTENSITY FORECAST IS ALSO ALIGNED WITH THE INTENSITY CONSENSUS
(ICNE). UNSURPRISINGLY, GFS SHOWS LITTLE DEVELOPMENT. HAFS-A
DEPICTS SIGNIFICANT INTENSIFICATION TO 95 KTS BY TAU 72, WHICH IS
NOT OUTSIDE THE REALM OF POSSIBILITY THAT TD 20W CAN QUICKLY
ORGANIZE, GIVEN THE SYSTEM SIZE. ALSO WORTH NOTING IS THE POTENTIAL
THAT TD 20W WILL DISSIPATE IN A FEW DAYS. THE INTENSITY FORECAST,
FOR NOW, IS SET NEAR THE CENTER OF THE GUIDANCE ENVELOPE WITH LOW
CONFIDENCE.

FORECAST CONFIDENCE:
   TRACK 00-72 HR: MEDIUM
   TRACK 72-120 HR: LOW
   INTENSITY 00-72 HR: MEDIUM
   INTENSITY 72-120 HR: LOW//
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